Return *descriptor.FieldDescriptorProto in FieldDescriptor

[Abbyyan]

According to https://github.com/golang/protobuf/issues/867, protobuf has supported to get message_type in field type. If there can return *descriptor.FieldDescriptorProto in types.gohttps://github.com/pseudomuto/protokit/blob/c63c13958542d70af1d1ef7826d30b25cd5bf6e9/types.go#L221, we can get the FileDescriptor of field whose type=method. Thanks a lot.