[Feature request] Inviting and accepting external users to rooms

[the-moog]

Here an 'external' user is one that is registered on a home server (HS) that is other than the local system.
Background:
Due to the way Matrix works, if an external user is invited to a room, the entire contents of that room, perhaps historically, is forwareded to that external HS. For some systems that may pose a GDPR, NDA, IP or security issue.

Obviously admins can create non-federated rooms, but that might not be the case as limited federation may be acceptable.

Suggestion:
If an externally registered user is invited to a room that is not e2e encrypted and is federated, then on invite, Riot should indicate via a user message that this exposes data with an option to cancel.

If a non-registered user is invited, e.g. via an email, then when they join a room if they are registered externally then another step should be inserted for the room admin to accept their request to join the room.

Enhancement:
How about a setting for a room of limited federation (rather than just on/off) where only certain extenral HS users can be invited?