Encrypted history sharing: in m.room_key messages, mark whether the room key can be reshared

[richvdh]

In order to know if a megolm session is safe to share to invited users as part of Encrypted history sharing, we need to annotate each session with a shared_history flag.

It is proposed that we add a field to the ExportedSessionData format.

We need to:

• set it when sharing keys over m.room_key
• store it in the db when receiving over m.room_key
• set it in the export file
• read from the export file
• set in key backup
• read from key backup
• ensure we rotate the key if the setting changes

The details of the flag should be the same as in matrix-org/matrix-spec-proposals#3061

[richvdh]

Todo: figure out what actually needs doing here and make some sub-issues

[BillCarsonFr]

• ensure we rotate the key if the setting changes

We are already checking for visibility changes to rotate keys:
https://github.com/matrix-org/matrix-rust-sdk/blob/d18669e8d9299ae363ff5f00cd6bb627bdf12846/crates/matrix-sdk-crypto/src/session_manager/group_sessions/share_strategy.rs#L174

 // This is calculated in the following code and stored in this variable.
    result.should_rotate = user_left || visibility_changed || algorithm_changed;

[poljar]

This has been done in matrix-org/matrix-rust-sdk#4700.