Support automatic FieldMasks with PATCH

[nemith]

GRPC-Gateway supports a PATCH feature¹ that when:

1. a non * body is provided
2. there is a update_mask field present that is of a type google.api.FieldMask
3. And the method is PATCH

Then the fields specified in the body are automatically added to the update path.

This is extremely useful for transcoding a idomatic REST interface to gRPC while stil supporting stanard gRPC interfaces.

1. https://grpc-ecosystem.github.io/grpc-gateway/docs/mapping/patch_feature/

This is needed for projects like envoy who use this library. (See envoyproxy/envoy#10235)