[Proposal][WIP] Messaging / FCM overhaul

[chrisbianca]

Messaging and FCM is an area that has fallen behind the rest of the library in terms of ease of use, documentation and reliability. We plan on overhauling this completely for the 3.3.0 release and rewriting from scratch to ensure that it provides the core FCM functionality in an easy to use and reliable manner.

This issue is currently a placeholder that will be updated as we agree exactly what we will support and how this will be structured in the API. It's also here to show that we're aware of the issues that have already been raised and will be addressing them as part of this work.

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