Please move MessageDifferencer to protobuf java

It should be there :) along with its native brother [google.protobuf.util.message_differencer](https://developers.google.com/protocol-buffers/docs/reference/cpp/google.protobuf.util.message_differencer)

[ProtoTruthMessageDifferencer.java](https://github.com/google/truth/blob/master/extensions/proto/src/main/java/com/google/common/truth/extensions/proto/ProtoTruthMessageDifferencer.java) -> [protocolbuffers/protobuf/tree/master/java](https://github.com/protocolbuffers/protobuf/tree/master/java)