Add support for RTL20

[AndyNicks]

Add support for the following requirements: RTL20

As stated at https://docs.ably.io/client-lib-development-guide/features/#RTL20

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

┆Issue is synchronized with this Jira Story by Unito

[jamienewcomb]

Closing this as we have decided we will not do this