Define interface for flat-file based message storage operations

Summary: This diff defines API in the form of Obj-C class that AppDelegate and NotificationService will use to write/retrieve messages temporarilt stored in a flat file.

Test Plan: No test plan. Future diffs that use this API in NotificationService and AppDelegate will provide appropriate test plan

Reviewers: palys-swm, varun, karol-bisztyga

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4391