Define interface for elementary file operations: atomic write, append, read and atomic truncation with encryption-decryption layer based on encryption key in CommSecureStore

Summary: Defines operations on file we will need to implement message stora in a flat file

Test Plan: No test plan.

Reviewers: palys-swm, varun, karol-bisztyga

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D4382