samples: add optimisticSubscribe, plus a few small library changes to support it

[feywind]

Adds a sample showing how to do optimistic subscription, to save on calls to the admin endpoint.

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/optimisticSubscribe.js:32, tag pubsub_optimistic_subscribe
• samples/typescript/optimisticSubscribe.ts:28, tag pubsub_optimistic_subscribe

---

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

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• README.md - README.md is managed by synthtool. However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/main/.readme-partials.yaml
• samples/README.md - samples/README.md is managed by synthtool. However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/main/.readme-partials.yaml