[lib][keyserver] Added new edit_message endpoint

Summary: Added new editMessageResponder with validation. Registered it as an endpoint.

Test Plan: In later diffs checked if it is possible to communicate with the keyserver. For now, checked if there are no flow errors and if the app is working correctly.

Reviewers: michal, inka, ginsu

Reviewed By: michal

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7128