Add support for RSC15e, d, f (Host Fallback )

[AndyNicks]

Add support for the following requirements: RSC15e, d, f

As stated at https://docs.ably.io/client-lib-development-guide/features/#RSC15e. https://docs.ably.io/client-lib-development-guide/features/#RSC15d. https://docs.ably.io/client-lib-development-guide/features/#RSC15f

see details https://github.com/ably/docs/blob/main/content/client-lib-development-guide/versions/features-1-1__1-2.diff

Acceptance Criteria:

Implement the support for the requirements above

Update the spreadsheet

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