chore: add default library settings

[parthea]

This PR adds default library settings for the proto package being generated to avoid KeyError when looking up the python specific settings.

Without this fix test test_read_empty_python_settings_from_service_yaml fails with the error below:

________________________________________________________________________________________ test_read_empty_python_settings_from_service_yaml ________________________________________________________________________________________

    def test_read_empty_python_settings_from_service_yaml():
        service_yaml_config = {
            "apis": [
                {"name": "google.example.v1beta1.ServiceOne.Example1"},
            ],
        }
        cli_options = Options(service_yaml_config=service_yaml_config)
        fd = get_file_descriptor_proto_for_tests(fields=[])
        api_schema = api.API.build(fd, "google.example.v1beta1", opts=cli_options)
>       assert api_schema.all_library_settings["google.example.v1beta1"].python_settings \
            == client_pb2.PythonSettings()
E       KeyError: 'google.example.v1beta1'

tests/unit/schema/test_api.py:2738: KeyError

[ohmayr]

Can you link me to this property in client_pb2 file?

[parthea]

https://github.com/googleapis/googleapis/blob/6a474b31c53cc1797710206824a17b364a835d2d/google/api/client.proto#L281

https://github.com/googleapis/python-api-common-protos/blob/f30115b7302afc91bf06f210a270a6530b7fafdf/google/api/client.proto#L281

googleapis/python-api-common-protos#249