Add special exception for CRDs generated by kubebuilder

See kubernetes-sigs/controller-tools#402
According to the schema the generated CRDs are invalid, but kubernetes
seems not to care.

examples/lint/kubebuilder.yaml

@@ -0,0 +1,74 @@
+
+---
+apiVersion: apiextensions.k8s.io/v1beta1
+kind: CustomResourceDefinition
+metadata:
+  annotations:
+    controller-gen.kubebuilder.io/version: v0.2.5
+  creationTimestamp: null
+  name: things.example.com
+spec:
+  additionalPrinterColumns:
+  - JSONPath: .metadata.creationTimestamp
+    name: Age
+    type: date
+  - JSONPath: .status.state
+    description: Status of the Thing.
+    name: State
+    type: string
+  group: environment.sapcloud.io
+  names:
+    categories:
+    - all
+    kind: Thing
+    listKind: ThingList
+    plural: things
+    singular: thing
+  scope: Namespaced
+  subresources:
+    status: {}
+  validation:
+    openAPIV3Schema:
+      description: Thing is a thing
+      properties:
+        apiVersion:
+          type: string
+        kind:
+          type: string
+        metadata:
+          type: object
+        spec:
+          description: ThingSpec defines the desired state of the Thing
+          properties:
+            color:
+              description: Color of the thing
+              enum:
+              - red
+              - green
+              - blue
+              type: string
+            category:
+              description: Category of the thing
+              type: string
+          type: object
+        status:
+          description: ThingStatus is the things status
+          properties:
+            state:
+              description: State of the thing
+              type: string
+          required:
+          - state
+          type: object
+      type: object
+  version: v1alpha1
+  versions:
+  - name: v1alpha1
+    served: true
+    storage: true
+status:
+  acceptedNames:
+    kind: ""
+    plural: ""
+  conditions: []
+  storedVersions: []

pkg/yttlint/validate.go

@@ -620,7 +620,7 @@ func (l *Linter) isSubset(defs v1.JSONSchemaDefinitions, subSchema, schema *v1.J
 					if l.Pedantic && !(magic.CouldBeString && !magic.CouldBeInt && !magic.CouldBeFloat) {
 						errors = append(errors, appendLocationIfKnownf(subSchema, `%s expected string got a computed value. Tip: use str(...) to convert to string`, path))
 					}
-				} else {
+				} else if path != ".metadata.creationTimestamp" { // https://github.com/kubernetes-sigs/controller-tools/issues/402
 					errors = append(errors, appendLocationIfKnownf(subSchema, "%s expected string got: %s", path, subSchema.Type))
 				}
 			}

pkg/yttlint/validate_test.go

@@ -60,6 +60,10 @@ func TestValidate(t *testing.T) {
 		filename:          "../../examples/lint/array-parameter.yaml",
 		nonPedanticErrors: []LinterError{},
 		pedanticErrors:    []LinterError{},
+	}, {
+		filename:          "../../examples/lint/kubebuilder.yaml",
+		nonPedanticErrors: []LinterError{},
+		pedanticErrors:    []LinterError{},
 	}, {
 		filename: "../../examples/lint/empty-pod.yaml",
 		nonPedanticErrors: []LinterError{{