Implement method to deserialize olm account from a file

Summary: This differential implements a method in NotificationsCryptoModule that can deserialize flat file containing JSON-fied pickled olm account into in-memory CryptoModule onject that is capable to perform end2end encrypted messaging sessions.

Test Plan:
Temporarily make this method public. Partially modify this method so that it does not read from a file but takes JSONI-ified pickled olm account directly and use it to create CryptoModule instance in CommCoreModule where we read pickled public olm account from the database. Use XCode debugger or logging to examine object
content. JSON-ified pickled olm account can be created in CommCoreModule.

Reviewers: tomek, bartek, kamil

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D6775