feat: add dead letter queue samples

[jdpedrie]

closes #1178.

[snippet-bot]

Here is the summary of changes.

You added 4 region tags.

• pubsub/api/src/dead_letter_create_subscription.php:26, tag pubsub_dead_letter_create_subscription
• pubsub/api/src/dead_letter_delivery_attempt.php:26, tag pubsub_dead_letter_delivery_attempt
• pubsub/api/src/dead_letter_remove.php:26, tag pubsub_dead_letter_remove
• pubsub/api/src/dead_letter_update_subscription.php:26, tag pubsub_dead_letter_update_subscription

---

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

[jdpedrie]

Thanks @anguillanneuf. This is fixed!