add proto-tools fromPb method?

[clairemcginty]

an example use case is inspecting the pipelineUrl file that Dataflow stages (which is a .pb file representing org.apache.beam.model.pipeline.v1.Pipeline) to verify coders and transforms. It would just be a wrapper around protoc's decode or decode_raw method, maybe with built-in support for common Protobuf messages like Pipeline (although we'd have to handle different schema versions for different Beam versions).