Improve BaseApiClient example

[llucax]

• Improve the typing of the stub: the mypy-protobuf plugin generates a XxxAsyncStub class for each service, which have proper type information to be used with async. This still needs some hacks (type ignore) to make it work, but it's better than using the normal stubs and having to cast on each call.
• Add channel_defaults.

[llucax]

Converting to draft because I'm finding issues at runtime with this approach, because the XxxAsyncStub doesn't really exist at runtime 😟

[llucax]

It looks like we need this accepted to make it work:

• Implement a new parameter to generate only async stubs nipunn1313/mypy-protobuf#631

Otherwise we can't use the AsyncStub as a generic parameter when inheriting from BaseApiClient.

If this doesn't get updated and we want to still use the async stub, I guess we need to take out the stub from the BaseApiClient class and each sub-class will need to handle it manually on their own.

[llucax]

A quick hack is:

    def __init__(...) -> None:
        super().__init__(
            server_url,
            microgrid_pb2_grpc.MicrogridStub,
            connect=connect,
            channel_defaults=channel_defaults,
        )
        self._async_stub: microgrid_pb2_grpc.MicrogridAsyncStub = self.stub  # type: ignore

And then just use self._async_stub internally.