[CommCoreModule] sign message with custom Olm Account

Summary:
ENG-9603.

Needed to sign a new device list with the old primary device account.

This doesn't need to be scheduled on cryptoThread because it is using the ephemeral CryptoModule, but perhaps it's better to keep it similar to all crypto-related operations and use a dedicated thread.

Depends on D14045

Test Plan: Tested in next diff in the stack

Reviewers: bartek, tomek

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D14046