Upgrade external evaluation message to proto3

[abeham]

Currently we have Google.ProtocolBuffers-2.4 by jskeet and Google.Protobuf 3.6.1 by Google's protobuf team. The later built upon the code by jskeet, but only supports proto3 syntax for C#.

The external evaluation message uses proto2 syntax and also uses extensions, which are not available in proto3 anymore. The protocol should thus be rewritten using proto3 syntax and using only the newer library. Tooling should be built upon the Google.Protobuf.Tools package.

There should be an easier way to return arbitrary data in the quality message.