Add support for disappearing messages (m.room.retention)

[milkcanworld]

Describe the problem

Though it is not part of the Matrix spec yet, Synapse (for example) has stable support for per-room message retention policies.

https://element-hq.github.io/synapse/latest/message_retention_policies.html

https://hyteck.de/post/matrix-forget/

Describe the solution you'd like

It would be nice if Sable had explicit support for setting the policy per-room, similar to Signal Messenger.

Alternatives considered

No response

Additional context

No response