This change adds a new HttpMessageConverter supporting
Google protocol buffers (aka Protobuf).
This message converter supports the following media types:
* application/json
* application/xml
* text/plain
* text/html (output only)
* and by default application/x-protobuf
Note, in order to generate Proto Message classes, the protoc binary
must be available on your system.
Issue: SPR-5807/SPR-6259
