Generate OpenAPI definitions

• To generate definition for a specific type or package add "+k8s:openapi-gen=true" tag to the type/package comment lines.
• To exclude a type or a member from a tagged package/type, add "+k8s:openapi-gen=false" tag to the comment lines.

OpenAPI Extensions

OpenAPI spec can have extensions on types. To define one or more extensions on a type or its member add +k8s:openapi-gen=x-kubernetes-$NAME:$VALUE`` to the comment lines before type/member. A type/member can have multiple extensions. The rest of the line in the comment will be used as $VALUE so there is no need to escape or quote the value string. Extensions can be used to pass more information to client generators or documentation generators. For example a type might have a friendly name to be displayed in documentation or being used in a client's fluent interface.

TODO(mehdy): Make k8s:openapi-gen a parameter to the generator now that OpenAPI has its own repo.