[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:

1. 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.
2. Notifs works.

Reviewers: ashoat, tomek

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D13486