Pub/Sub User Guide - Subscriber

[dbolduc]

These are user guide chapters / examples we want to show for using a Subscriber client.

#[non_exhaustive]
// TODO : include the necessary Dev/Rel snippets.
// @suzmue compiled a list of candidates in:
// https://github.com/googleapis/google-cloud-rust/issues/3017#issuecomment-3293254090

• [ ] Quickstart (pubsub_quickstart_subscriber)
• [ ] Using the streaming API as a callback