[lib] implement fetching missing peers when generating P2P messages
Summary:
ENG-9394
Similar to D13443 but more generic solution to handle all type of DM ops.
Depends on D13485
Test Plan:
Tested later in the stack but:
- Making sure that sending text messages or thread creation (or any other DM ops) to a peer that is not in the aux user store is delivered.
- Notifs works.
Reviewers: ashoat, tomek
Reviewed By: ashoat
Differential Revision: https://phab.comm.dev/D13486