Add ephemeral messages support (MSC2228) (#6409)

Implement part [MSC2228](https://github.com/matrix-org/matrix-doc/pull/2228). The parts that differ are:

* the feature is hidden behind a configuration flag (`enable_ephemeral_messages`)
* self-destruction doesn't happen for state events
* only implement support for the `m.self_destruct_after` field (not the `m.self_destruct` one)
* doesn't send synthetic redactions to clients because for this specific case we consider the clients to be able to destroy an event themselves, instead we just censor it (by pruning its JSON) in the database