docs(sample): Add samples for topic and subscription SMTs

[kamalaboulhosn]

No description provided.

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/snippets/publisher.py:334, tag pubsub_create_topic_with_smt
• samples/snippets/subscriber.py:585, tag pubsub_create_subscription_with_smt

---

This comment is generated by snippet-bot.
If you find problems with this result, please file an issue at:
https://github.com/googleapis/repo-automation-bots/issues.
To update this comment, add snippet-bot:force-run label or use the checkbox below:

• Refresh this comment

[mukund-ananthu]

I see the samples presubmits failing with

    @functools.wraps(callable_)
    def error_remapped_callable(*args, **kwargs):
        try:
            return callable_(*args, **kwargs)
        except grpc.RpcError as exc:
>           raise exceptions.from_grpc_error(exc) from exc
E           google.api_core.exceptions.InvalidArgument: 400 You have passed an invalid argument to the service (argument=message_transforms).

Blocking the PR merge. I'll take a look at this later today / tomorrow.