feat: microgen - adds two partial templates for creating method signatures

[chalmerlowe]

• Follows: PR feat: microgen - adds two init file templates #2286 (should be merged after that PR is merged.)

🛑 WARNING Github automagically swapped the base branch. This was supposed to merge into the branch in PR #2287 (already merged), but Github changed things which somehow added the _helpers.py.j2 file from #2287 to this PR. That file has already been merged so there is no need to review it. 🛑

The focus of your code review only needs to be these two files:

This PR adds two templates used to generate action methods on the centralized client class (e.g. get_dataset(), list_tables(), delete_routine())

These two templates contemplate two different scenarios where an action method is:

• expected to be able to accept a string as a path_identifier (e.g. project_id.dataset_id.model_id) > see file: _method_with_request_builder.j2
• NOT expected to be able to accept a string as a path_identifier > see file: _simple_passthrough_method.j2

The following is an example of what _method_with_request_builder.j2 might be used to generate:

def get_dataset(
    self,
    dataset_id: Optional[str] = None,
    *,
    request: Optional[dataset.GetDatasetRequest] = None,
    retry: OptionalRetry = DEFAULT_RETRY,
    timeout: Union[float, object] = DEFAULT_TIMEOUT,
    metadata: Sequence[Tuple[str, Union[str, bytes]]] = DEFAULT_METADATA,
) -> "dataset.Dataset":
    """
    TODO: Docstring is purposefully blank. microgenerator will add automatically.
    """
    if request and dataset_id is not None:
        raise ValueError("Cannot provide both request and dataset_id.")

    if request:
        final_request = request
    else:
        path_identifier = dataset_id
        if path_identifier is None:
            if len(['project_id', 'dataset_id']) == 1:
                path_identifier = self.project
            else:
                raise ValueError("Either request or dataset_id must be provided.")

        if path_identifier is None:
            raise ValueError("Could not determine a path identifier.")

        request_class = dataset.GetDatasetRequest

        final_request = _helpers._create_request(
            request_class=request_class,
            path_identifier=path_identifier,
            expected_args=['project_id', 'dataset_id'],
            default_project_id=self.project,
        )

    return self.dataset_service_client.get_dataset(
        request=final_request,
        retry=retry,
        timeout=timeout,
        metadata=metadata,
    )

[chalmerlowe]

For clarity:

1. The GitHub Actions are being used to help ensure that unit tests pass.
   
   
2. The KOKORO tests are failing. This is a known problem and will be dealt with in a separate PR. It should not affect merging into the autogen dev branch.