ably-go doesn't retry requests to fallback hosts on a timeout

[mclark-ably]

https://sdk.ably.com/builds/ably/specification/main/features/#RSC15l specifies the the conditions of when an Ably SDK should retry a request to a fallback endpoint.

Specifically https://sdk.ably.com/builds/ably/specification/main/features/#RSC15l2 is not implemented. A request that times out continues to try at the primary endpoint e.g.

[ERROR] RestClient: failed sending a request Post \"https://rest.ably.io:443/keys/<Ably_key_name>/requestToken\": context deadline exceeded"}

[sync-by-unito]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-4024

[sacOO7]

This also needs to be refactored based on one of the old fallbacks PR
https://github.com/ably/ably-go/pull/316/files