[lib] Create content Olm session

Summary:
Use the keyserver keys to create the sessions.

Depends on D10667

Test Plan: Get access token, set it, and check if the session is created correctly... which returns a bad signature error due to https://linear.app/comm/issue/ENG-6468/decide-on-one-time-keys-format

Reviewers: varun, inka, kamil

Reviewed By: varun

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10777