add support for compiling from a file registry

[marcoferrer]

We're testing out hyperpb in one of our existing use cases. The issue we're running into is that we already have a protoregistry.Files instance, which means we end up having to do an additional conversion back to FileDescriptorSet before hyperpb converts that back into an instance of protoregistry.Files internally.

[CLAassistant]

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