Implement translation of message JSON object into SQLite database entities

Summary: Implement function that translates JSON object representing notification content into Message and Media database entities

Test Plan: No testing plan. Future diffs will provide a code that uses those functions on iOS and Android respectively

Reviewers: palys-swm, karol-bisztyga

Reviewed By: palys-swm, karol-bisztyga

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D3983