Audit the developer experience of using the SDK in Swift concurrency

[lawrence-forooghian]

I recently tried, for the first time, using ably-cocoa with Swift concurrency (whilst working on the Chat SDK). I had previously just assumed that the Swift compiler would bridge the callback-taking methods like attach(_:) to an async equivalent, but it doesn’t. This means that users have to write boring boilerplate with withCheckedThrowingContinuation. Let's see what we can do to improve this experience.

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