make it possible to decode LRO response and metadata

[alexander-fenster]

This might be a combined issue in the generator and google-gax.

Users often need to get the LRO result and metadata knowing operation name, and we don't provide an API for that (it's possible but requires some manual proto decoding).

Example of this: googleapis/nodejs-speech#399

Would be great to have a good API for LRO calls that will do this. Maybe an overload for all LROs that accept an operation name. Would be great to consult other languages and see what they do.