Add model name generator

[jpbetz]

This moves from a reflection based approach to OpenAPI model naming to a declarative approach.

OpenAPIModelName() receiver functions to access OpenAPI model names of API types are generated into zz_generated.model_name.go files. This allows API authors to declare the desired OpenAPI model packages when the model name derived from the go package path is not desirable.

This PR also migrates the kubernetes API to use this generator (all OpenAPI model names remain the same).

For example:

// doc.go

// +k8s:openapi-gen=true
// +k8s:deepcopy-gen=package
// +k8s:protobuf-gen=package
// +k8s:prerelease-lifecycle-gen=true
// +k8s:openapi-model-package=io.k8s.api.core.v1

// +groupName=
package v1

Generates:

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

[jpbetz]

@BenTheElder @Jefftree @thockin

I've moved the codegen part of this into openapi-gen. This is a backward compatible change that enables us to switch away from go package name reflection in k8s.

[thockin]

/lgtm
/approve

I am not intimately familair with this generator but this PR all seems reasonable to me :)

[thockin]

Why not output to the same file?

EDIT: Oh I see. 1 Generator -> 1 file. Probably not worth changing, but IMO outputting to a single file would be cleaner, if we run across similar things in other generators. Unless there's a reason this is better that I am missing.

[jpbetz]

There is one file for the generated openapi definition, and there are N files for all the zz_geneated.model_name.go files, one for each package containing types that need this function added. Those files are very similar to deep copy generated files.

[thockin]

OH RIGHT. Nevermind me :)

[k8s-ci-robot]

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This pull-request has been approved by: jpbetz, thockin

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