Add new properties to `ARTMessage` and add/update appropriate docstrings

[maratal]

The following properties were added recently: action, version, serial, refSerial, refType, operation, createdAt

action and serial were added as part of https://github.com/ably/ably-cocoa/pull/1998, but need update after adding the rest of the fields.

https://sdk.ably.com/builds/ably/specification/main/features/#TM2

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