[backend] [feat] [0/n] add update settings endpoint

Summary:
Add update settings API endpoint to backend.

• Adds create or update user settings updater
• Create API route
• UpdateUserSetting Types

Test Plan: Test with the entire stack. MySQL DB settings table should update via the native UI (default notifications in profile screen) when clicking "all", "background", or "none"

Reviewers: palys-swm, ashoat

Reviewed By: ashoat

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, boristopalov

Differential Revision: https://phabricator.ashoat.com/D2276