Add support for basic gRPC API Configuration YAML files for gRPC-JSON transcoder

Sometimes the requirement to have to annotate our .proto files to expose them can be inconvenient. Google cloud provides an alternative approach where we specify our HTTP to gRPC mappings in gRPC API Configuration YAML file rather than in .proto. So is configuration with yaml files supported for gRPC-JSON transcoder in envoy? Or maybe it will be supported later?