Generate `__init__.py` files in `py/` mirror `proto/`

[Marenz]

When generating the python files from proto files, protobuf is not creating missing __init__.py files, so nested modules are not considered proper Python modules/packages.

During the generation we could check for any missing __init__.py files and automatically generate them as part of the generation process.

[llucax]

We've been bitten by this again:

• Rename v1 to v1alpha7, and use v1 from v0.6.x frequenz-api-common#374

It doesn't happen often, but when it does it tends to take quite a bit of time to find out what the issue is, so increasing the priority.