Option to only share presence with members of rooms with `m.direct`

This issue has been migrated from [#11977](https://github.com/matrix-org/synapse/issues/11977).

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As you know, presence updates have a major impact on the overall server performance... but just turning it off is not a real solution in most use-cases. Therefore, I'm suggesting a new option in the presence configuration called `limit_to_direct_rooms` (or something like that).

The idea is, that when collecting the list of interested parties for presence updates, only room ids with a `m.direct` membership should be used. As a result, users would retrieve presence updates for their "contacts", but not for everyone who's sharing a group chat room. This could help decreasing the server load and increase privacy at the same time.

What do you think? Could this be a reasonable solution?