Provide an easy way to add/remove ably channel state listeners

[AndyTWF]

In #963, we had to introduce a local boolean check in two places to prevent duplicate resumptions of continuations related to ably channel state changes (the duplicates call exceptions to be thrown).

This was necessary because in Ably.kt (DefaultAbly) we wrap the standard Ably state change listeners in our own classes, meaning we cannot get the object that's registered with ably to be able to call channel.off(). We also have to send the "initial" state before the listener is attached to the channel, which presents a race condition. Changing this would require some careful thinking, so in the interest of time we added the work-around to solve the issue in the short term.

We should look to go back and readdress this so that we use the standard channel.off() way of unsubscribing listeners (and listeners can unsubscribe themselves) in the future.

[AndyTWF]

Note this code has now been moved into Ably.kt, but remains the same principle.

[deanna-lad]

The corresponding Jira issue is https://ably.atlassian.net/browse/SDK-3291

[sync-by-unito]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-3291