Historical room key sharing (key share on invite)

[richvdh]

When Alice invites Bob to a room with a suitable history visibility setting, she is supposed to share any keys she has for past messages in that room (via m.forwarded_room_key events, I think).

(Aside: I don't think this is actually specced?)

Anyway: this needs implementing in Element-R.

[richvdh]

See also #23778

[richvdh]

(Aside: I don't think this is actually specced?)

This is matrix-org/matrix-spec#1655, for the record.

[richvdh]

Requires implementation in matrix-rust-sdk (matrix-org/matrix-rust-sdk#580)