google protobufs dependency

[mjheilmann]

This module uses :google_protobufs in some of its tests, which now interacts with consuming classes. Can we have good protobuf testing without using external modules containing their own definitions?

[mjheilmann]

in examples/helloworld we added a git submodule for the google protobufs and generated the types directly while checking for cross support between protobuf_elixir and protobuf_generate. Since we generate our own types in the example, and for our test modules, we can do so for the test modules as well. This would allow us to reduce our package dependencies as a library