Support marshalling messages from human-readable format

[bradleyjkemp]

When writing fixtures or writing dumps to be replayed for testing purposes, it would be much nicer to be able to write messages in the human readable form rather than requiring raw bytes.

grpc-replay and grpc-fixture should marshal the human readable format and send this instead of reading the raw_message field.

At the moment JSON Proto parsing doesn't support unknown fields so I think for now this will only be possible when proto definitions have been loaded.

Perhaps messages that have been heuristically decoded could be tagged and a custom https://github.com/jhump/protoreflect based parsed be used for when we don't have protos?