Support custom JSON marshaller for error details in unframed grpc error

[cormoran]

In short, implements this TODO comment.

When using grpc richer error response, we sometimes want to pass custom type in Status.details instead of well-known details.
Now it fails for unframed http transcoding requests since the error marshaller doesn't know custom types and there is no way to register custom types.

[ikhoon]

It is time to add a builder class to create UnframedGrpcErrorHandler with various options.
For example:

UnframedGrpcErrorHandler.builder()
                        .responseType(UnframedGrpcErrorResponseType.JSON,
                                      UnframedGrpcErrorResponseType.PLAINTEXT)
                        .statusFunction(UnframedGrpcStatusMappingFunction)
                        .jsonMarshaller(MessageMarshaller)
                        .build()