Implement grpc-gateway's patch feature for GrpcJsonTranscoder

grpc-gateway [implements](https://grpc-ecosystem.github.io/grpc-gateway/docs/patch.html) Google's ["Standard API" Update method](https://cloud.google.com/apis/design/standard_methods#update), which we use w/ grpc-gateway. We'd love to switch to Envoy to reduce the complexity of our services but this is holding us back. Is this functionality already implemented, or is this a feature request?