Improve and fix support for compiling protobuf files

[leandro-lucarella-frequenz]

The main changes are:

• Avoid running sessions on sources for API repos

  API repos are supposed to just ship python files generated from protocol buffer files. Only a stub __init__.py should be included, which doesn't really need to be checked.

  Allowing checks on the sources for an API repos triggers a lot of issues when the protocol buffers files are generated, as they don't pass any checks. An alternative would be to exclude auto-generated files, but it's not easy because each tool has its own exclusion mechanism, and some don't even have any (hello darglint), so it is simpler to just skip testing the stub __init__.py file.

• Avoid the need to write a setup.py file for API repos

  To do this we declare a distutils entry point in the pyproject.toml file to automatically add the compile_proto command using the CompileProto proto class.

  Then, in the grpc_tools module we just add the newly declared command as a build sub-command, so it is executed first. This make all other subsequent sub-commands will see the generated sources as if they were already there.

  The only downside of this is now we don't have an easy way to configure the command. The options are there, and can be passed via command-line, like:

  python -c 'import setuptools; setuptools.setup()' compile_proto \
      --proto-path=another/path

  But there is no easy way to do it in a config file, or at least at the moment I couldn't find a way to pass arbitrary options to setuptools commands in pyproject.toml. If we need this, we could manually parse the pyproject.toml file and look for options, just like setuptools_scm does.

[leandro-lucarella-frequenz]

Incidentally, while looking at the microgrid API repo to see how it was doing this, I realized the CI is not really testing anything. It uses tox and it is run without specifying a session, which means tox runs some py sessions that doesn't do anything 🎉 🤦

[leandro-lucarella-frequenz]

With this I was able to migrate the dispatch API repo to use this one.

[leandro-lucarella-frequenz]

OK, no complains for a month now, so force-merging 💪