Message retention implementation tracking

[mredig]

In implementation for https://github.com/matrix-org/matrix-spec-proposals/blob/matthew/msc1763/proposals/1763-configurable-retention-periods.md, I intend to break this up into at least two PRs:

1. endpoint retrieval and event/state handling
2. execution of removing expired messages per retention policy

This issue is just for tracking the bigger task, re @stefanceriu's suggestion.

[Hywan]

Thanks!

That's a good idea. Feel free to ping us whenever you need assistance.