Populate feature compliance for `Realtime: Authentication: Get Confirmed Client Identifier`

[QuintinWillison]

Added to the canonical feature list in https://github.com/ably/features/pull/19.

At that time it wasn't a trivial task for me to confirm this functionality was implemented correctly, according to the spec, in this SDK.

Until the manifest for this SDK has been moved to this repository, the work to be done under this issue will need to be made in the manifest file over in the features repository's incubation / prototypes folder.

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