Make protobuf files accessible publicly

[olivierbeaulieu]

Python and Go guides use gRPC, which depends on the protobuf files. Currently, the examples reference public repos that contain a copy of the protobuf files that may or may not be out of date.

A solution would be to create a public repo that would become the source of truth for protobuf files.

Furthermore, a user going through the guides right now need to have protoc installed and compile the protobufs themselves. Ideally, we should provide compiled protobufs in this repo as well, so that we can simplify the guides. The user would simply wget the compiled files, and carry on with the rest of the tutorials.

cc @billettc