gRPC Gateway query

The gwquery package contains a slightly modified copy of the query.go file from gRPC Gateway, copied from github.com/grpc-ecosystem/grpc-gateway/blob/main/runtime/query.go, and is licensed according to ./LICENSE. The parseField method is modified to avoid depending on the global protoregistry, which doesn't work in grpcbridge's case where each target has its own registry.

TODO(renbou): push a fix upstream once this is tested to properly work with grpcbridge