protocmp: support custom Resolvers in Transform

[tommie]

Is your feature request related to a problem? Please describe.

When using Any in unit tests for a proxy, I want to be able dynamically generate Protobuf definitions, load them using dynamicpb and compare them with protocmp. In this case, I read the FileDescriptorSet from storage rather than having them compiled in.

Describe the solution you'd like
There's already a TODO in handling Any suggesting support for a custom resolver. I think this is a good idea.

My proposal is to simply add a MessageTypeResolver(protoregistry.MessageTypeResolver) option to Transform. It requires creating a transformer struct, and making many of the (unexported) functions members of it. See tommie/protobuf-go@0a7aeae for a PoC.

Describe alternatives you've considered
Just having a "testing.proto" where I put all the various protos I need for testing. In my case, the FileDescriptorSet is generated together with other information, and requiring one path for production and one for testing is inconvenient.

[neild]

This seems reasonable to me.

cc @joetsai