Expose response metadata from access API responses

[peterargue]

This is part of #736.

All of the Access API endpoints include a metadata blob in the response that includes details about the access nodes from the time that the response was generated:
https://github.com/onflow/flow/blob/master/protobuf/flow/entities/metadata.proto

Make this info available to users of the API.

I don't think the answer should be to include it as a member of every response struct. We will want to come up with a was to expose it that's convenient and doesn't create a ton of complexity or tech debt

[peterargue]

We're going to skip this work for now since we have not received any requests to add it and how it should be exposed is not clear