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Move unstructured conversion into pkg/runtime

Scheme conversion should support unstructured conversion natively to
allow going from unstructured to typed and back. It is not a higher
level responsibility to do that conversion because the scheme is the
only one who knows what types it supports.

@liggitt @kubernetes/sig-cli-api-reviews I am going to make Scheme support unstructured in ConvertToVersion and Convert, which means resource.Builder and the CLI can get simpler for all existing use cases where versioned and unstructured need to coexist.