Define message spec interface and implement it for each message type that produces notifications

Summary: Implement message spec interface for each type of message that mirrors corresponding JavaScript implementation.

Test Plan: No testing plan. Guture diffs will provide a code that was used on Android and iOS to parse raw message string and store it to database.

Reviewers: palys-swm, karol-bisztyga, ashoat

Reviewed By: palys-swm, karol-bisztyga, ashoat

Subscribers: ashoat, Adrian, atul, yayabosh

Differential Revision: https://phab.comm.dev/D3940