[CommRustModule] implement getInboundKeys

Summary:
There is some code duplications between this and getOutboundKeys but I am afraid trying to unify this will make the code more messy.

Depends on D10373

Test Plan:

1. Login to staging Identity (loginPasswordUser) on two devices using the same account.
2. Call this method and check if the result is correct.

Reviewers: marcin, varun, michal

Reviewed By: varun, michal

Subscribers: marcin, ashoat, tomek

Differential Revision: https://phab.comm.dev/D10374