Calculate required response header parameters for initialize (#150)

### Motivation

Resolve #30

### Modifications

Define a computed property on StructBlueprint that tells you whether the
struct has an empty initializer, and it is used when response header
struct blueprint is created.

### Result

If one or more required response header parameters are present,
Generates code that needs to set request header parameters when create
response.

For example you define the yaml below

```yaml
      responses:
        '200':
          description: A success response with a greeting.
          headers:
            X-Extra-Arguments:
              required: false
              description: "A description here."
              content:
                application/json:
                  schema:
                    $ref: '#/components/schemas/Greeting'
```

`Types.swift` is generated like below, and you can set the "headers"
parameter if necessary ,

<img width="765" alt="スクリーンショット 2023-07-29 14 44 45"
src="https://github.com/apple/swift-openapi-generator/assets/16571394/c8e5779a-e851-40d6-9c68-9ced8af4dcc2">


if you set required true
```yaml
            X-Extra-Arguments:
              required: true

```

`Types.swift` is generated like below, and you should set reponse
headers when initialize

![スクリーンショット 2023-07-29 14 52
55](https://github.com/apple/swift-openapi-generator/assets/16571394/33890627-126e-4d94-af89-040943d2b489)

### Test Plan

Add test optional response header in SnipetBasedReferenceTests, and
modify ReferenceSource a little.

---------

Co-authored-by: Honza Dvorsky <honza@apple.com>
Co-authored-by: Si Beaumont <simonjbeaumont@gmail.com>

Author: 3 people

Sources/_OpenAPIGeneratorCore/Translator/CommonTypes/StructBlueprint.swift

@@ -80,6 +80,21 @@ struct StructBlueprint {
     var properties: [PropertyBlueprint]
 }
 
+extension StructBlueprint {
+
+    /// A Boolean value indicating whether the struct can be initialized using
+    /// an empty initializer.
+    ///
+    /// For example, when all the properties of the struct have a default value,
+    /// the struct can be initialized using `Foo()`. This is important for
+    /// other types referencing this type.
+    var hasEmptyInit: Bool {
+        // If at least one property requires an explicit value, this struct
+        // cannot have an empty initializer.
+        properties.allSatisfy { $0.defaultValue != nil }
+    }
+}
+
 /// A structure that contains the information about an OpenAPI object property
 /// that is required to generate a matching Swift property.
 struct PropertyBlueprint {

Sources/_OpenAPIGeneratorCore/Translator/Responses/translateResponse.swift

@@ -36,20 +36,21 @@ extension TypesFileTranslator {
         let headerProperties: [PropertyBlueprint] = try headers.map { header in
             try parseResponseHeaderAsProperty(for: header)
         }
+        let headersStructBlueprint: StructBlueprint = .init(
+            comment: nil,
+            access: config.access,
+            typeName: headersTypeName,
+            conformances: Constants.Operation.Output.Payload.Headers.conformances,
+            properties: headerProperties
+        )
         let headersStructDecl = translateStructBlueprint(
-            .init(
-                comment: nil,
-                access: config.access,
-                typeName: headersTypeName,
-                conformances: Constants.Operation.Output.Payload.Headers.conformances,
-                properties: headerProperties
-            )
+            headersStructBlueprint
         )
         let headersProperty = PropertyBlueprint(
             comment: .doc("Received HTTP response headers"),
             originalName: Constants.Operation.Output.Payload.Headers.variableName,
             typeUsage: headersTypeName.asUsage,
-            default: headerProperties.isEmpty ? .emptyInit : nil,
+            default: headersStructBlueprint.hasEmptyInit ? .emptyInit : nil,
             associatedDeclarations: [
                 headersStructDecl
             ],

Tests/OpenAPIGeneratorReferenceTests/Resources/ReferenceSources/Petstore/Types.swift

@@ -686,7 +686,7 @@ public enum Components {
             ///   - headers: Received HTTP response headers
             ///   - body: Received HTTP response body
             public init(
-                headers: Components.Responses.ErrorBadRequest.Headers,
+                headers: Components.Responses.ErrorBadRequest.Headers = .init(),
                 body: Components.Responses.ErrorBadRequest.Body
             ) {
                 self.headers = headers
@@ -1006,7 +1006,7 @@ public enum Operations {
                 ///   - headers: Received HTTP response headers
                 ///   - body: Received HTTP response body
                 public init(
-                    headers: Operations.createPet.Output.Created.Headers,
+                    headers: Operations.createPet.Output.Created.Headers = .init(),
                     body: Operations.createPet.Output.Created.Body
                 ) {
                     self.headers = headers

Tests/OpenAPIGeneratorReferenceTests/SnippetBasedReferenceTests.swift

@@ -519,7 +519,7 @@ final class SnippetBasedReferenceTests: XCTestCase {
         )
     }
 
-    func testComponentsResponsesResponseWithHeader() throws {
+    func testComponentsResponsesResponseWithOptionalHeader() throws {
         try self.assertResponsesTranslation(
             """
             responses:
@@ -541,6 +541,42 @@ final class SnippetBasedReferenceTests: XCTestCase {
                     public var headers: Components.Responses.BadRequest.Headers
                     @frozen public enum Body: Sendable, Equatable, Hashable {}
                     public var body: Components.Responses.BadRequest.Body?
+                    public init(
+                        headers: Components.Responses.BadRequest.Headers = .init(),
+                        body: Components.Responses.BadRequest.Body? = nil
+                    ) {
+                        self.headers = headers
+                        self.body = body
+                    }
+                }
+            }
+            """
+        )
+    }
+
+    func testComponentsResponsesResponseWithRequiredHeader() throws {
+        try self.assertResponsesTranslation(
+            """
+            responses:
+              BadRequest:
+                description: Bad request
+                headers:
+                  X-Reason:
+                    schema:
+                      type: string
+                    required: true
+            """,
+            """
+            public enum Responses {
+                public struct BadRequest: Sendable, Equatable, Hashable {
+                    public struct Headers: Sendable, Equatable, Hashable {
+                        public var X_Reason: Swift.String
+                        public init(X_Reason: Swift.String) {
+                            self.X_Reason = X_Reason }
+                    }
+                    public var headers: Components.Responses.BadRequest.Headers
+                    @frozen public enum Body: Sendable, Equatable, Hashable {}
+                    public var body: Components.Responses.BadRequest.Body?
                     public init(
                         headers: Components.Responses.BadRequest.Headers,
                         body: Components.Responses.BadRequest.Body? = nil