Add OpenAPIModelNamer and opt-in generator support

[jpbetz]

This enables kube-openapi to use model names defined by the models instead of the <go-package>.<go-type-name> of the Go structs that defines the model.

This is entirely opt-in. Existing behavior is retained.

openapi-gen users may opt-in with the --use-openapi-model-names.

Models may opt-in by implementing the OpenAPIModelNamer interface introduced by this change:

type OpenAPIModelNamer interface {
	OpenAPIModelName() string
}

For example:

func (in Pod) OpenAPIModelName() string {
	return "io.k8s.api.core.v1.Pod"
}

When used, the "canonical type names" and "REST friendly names" are the same model name.

This is simpler than the existing behavior where the name formats differ needlessly require conversion:

• Canonical type names
  • e.g. k8s.io/apiextensions-apiserver/pkg/apis/apiextensions/v1.CustomResourceDefinition
• REST friendly names
  • e.g. io.k8s.apiextensions-apiserver.pkg.apis.apiextensions.v1.CustomResourceDefinition

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: jpbetz

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[jpbetz]

/assign @Jefftree
cc @aojea @BenTheElder
/sig api-machinery
/triage accepted

[Jefftree]

Did you encounter one of these types or is this just precautionary?

[jpbetz]

int64Amount is leaking out from quantity into pkg/generated/openapi/zz_generated.openapi.go. A grep will show this. runtime.Object also shows up in that file.

[Jefftree]

/lgtm
/hold

in case you want additional eyes