Start NotificationsCryptoModule API definition and implementation

Summary: This differential launches definition and implementation of NotificationsCryptoModule C++ API. Foundational methods to serialize and deserialize file containing notifications olm account are defined. Relevant build files on both platforms are also modified.

Test Plan: There is not test plan for this differential. Header files are not compiled if there is no cpp file including them. Very next differentials are going to include this file and implement methods from this file.

Reviewers: tomek, bartek, atul, jon

Reviewed By: bartek, jon

Subscribers: jon, ashoat

Differential Revision: https://phab.comm.dev/D6774