Add application/vnd.google.protobuf to ProtobufCodecSupport

[making]

While RSocket spec supports application/vnd.google.protobuf as a Well-known MIME Type, Protobuf Codec doesn't.
As a result, Spring RSocket app that has ProtobufDecoder configured can not receive a payload with application/vnd.google.protobuf data mime type.
https://github.com/rsocket/rsocket/blob/master/Extensions/WellKnownMimeTypes.md

[bclozel]

@making application/vnd.google.protobuf sounds good, but is "application/octet-stream" really necessary? This one might be problematic.

[making]

I think it's safe to keep it for the backwards compatibility.

[sbrannen]

@making application/vnd.google.protobuf sounds good, but is "application/octet-stream" really necessary?

"application/octet-stream" was already there.

[bclozel]

Ha, never mind I misread the diff. Thanks!

[rstoyanchev]

I think this safe to do. It would expand the capability on the decoding side, but on the encoding side we still default to the first one on the list.