[lib] Added new MessageSpec for editing messages

Summary: Added new MessageSpec for editing messages. Registered it in the MessageSpecs.

Test Plan: Checked if code compiles. Run app on the web and native. Then added new message with new type to database, and checked if the app still works, and loads properly the new message type by logging info to the console. Couldn't check yet all scenarios (like creating new message type locally on client side)

Reviewers: michal, inka, ginsu

Reviewed By: ginsu

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6963