Migrate to using Swift concurrency

[lawrence-forooghian]

Now that we've confirmed a deployment target of iOS 13 (#597), we can start using Swift concurrency. As well as async/await and structured concurrency, this also allows us to use AsyncSequence, which we may wish to start using as a way of achieving parity with Android’s use of Kotlin streams.

We can start using these features internally immediately, and may wish to refactor existing internal code to use it. We should also consider migrating the public API to use them.

This issue serves to remind us of this opportunity, and we can split it out into separate more detailed tickets as we wish.

[sync-by-unito]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-3450