Create a new `client` repository type

[llucax]

What's needed?

The API clients will be soon split to their own repository, but there is no template to handle this use case completely, where we have a repository that contains no proto files (or all of them come from sub-modules) and have both code generated from those proto files but also manually written code.

Proposed solution

• Create a new repository type client that handles correctly this new use case.
• Add cross-referencing for gRPC: https://grpc.github.io/grpc/python/objects.inv

Use cases

All new client repositories.

Alternatives and workarounds

No response

Additional context

Reported by @shsms

It turns out in api clients, mypy etc isn't running, because repo-config excludes the py directory. But that's where we're putting the client code. Can you think of any quick workaround?