[Runtime] Choose the serialization method based on content type (#12)

[Runtime] Choose the serialization method based on content type

### Motivation

Resolves apple/swift-openapi-generator#43. This PR also resolves apple/swift-openapi-generator#42. It's paired with apple/swift-openapi-generator#48.

This PR introduces the initial content-type-aware coding strategy, which is likely to get expanded in the future.

### Modifications

This PR covers the scope described in issue 43 - handling a `type: string` appropriately, based on whether it's wrapped in `application/json` or `text/plan` content.

It introduces new variants of the SPI helper functions, which now has variants per "coding strategy". It's best to read https://github.com/apple/swift-openapi-generator/pull/48/files?short_path=aa92e58#diff-aa92e58fb5a311512cfcd285827a4aa2e6634cae7fdfe0090bd2cfc7b0986140 for details.

Deprecated all of the SPI methods that didn't contain the coding strategy parameter, to be removed in the next breaking version.

### Result

We now have explicit control over the coding strategy for ambiguous types, such as `String`, which conform to both `Codable` and `LosslessStringConvertible`, to be taken advantage of by the generator (see the paired PR).

### Test Plan

Verified that the default behavior now matches version 0.1.0, and when paired with the updated generator, fully resolves issue 43.

Used code coverage to ensure that `Converter+{Common,Client,Server}.swift` are now 100% unit tested.


Reviewed by: simonjbeaumont

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#12

Author: czechboy0

Sources/OpenAPIRuntime/Base/EncodableBodyContent.swift

@@ -14,16 +14,22 @@
 
 /// A wrapper of a body value with its content type.
 @_spi(Generated)
-public struct EncodableBodyContent<T: Encodable & Equatable>: Equatable {
+public struct EncodableBodyContent<T: Equatable>: Equatable {
 
     /// An encodable body value.
     public var value: T
 
     /// The header value of the content type, for example `application/json`.
     public var contentType: String
 
-    /// Creates a new content wrapper around the specified value and content type.
-    public init(value: T, contentType: String) {
+    /// Creates a new content wrapper.
+    /// - Parameters:
+    ///   - value: An encodable body value.
+    ///   - contentType: The header value of the content type.
+    public init(
+        value: T,
+        contentType: String
+    ) {
         self.value = value
         self.contentType = contentType
     }