[CommCoreModule] implement decrypting content with olm session

Summary:
Implemented method to decrypt content.

Depends on D10377

Test Plan:
This code is really hard to test as it's impossible to verify partial results.
I generated some keys and values, verified them with olm-utils.test.js, and checked if they succeeded/failed when calling the method introduced here in the same cases as JS tests.
Full functionality tested later in the stack.

Reviewers: marcin, varun, michal

Reviewed By: marcin

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D10378