[native] make it possible to test signing using Olm Account from User Keys

Summary:
ENG-9603.

This is a test case and an example of how to use it, probably this code can be updated, exported, and used in Restore Flow later (cc. @tomek).

This is a real-life use case because we take the previous Primary Device deviceID from the device list history, then sign the message using Olm Account from Backup without extracting deviceID, and use deviceID from the device list to verify — so if this passes verification, both device list history and User Keys upload/download/crypto operations work together.

Depends on D14046

Test Plan: Perform what is explained in test case

Reviewers: bartek, tomek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D14047