[keyserver] Introduce updateUserAvatarResponder endpoint scaffolding

Summary:
Introduce basic scaffolding so we can hit update_user_avatar endpoint on `keyserver. Next diff will introduce client scaffolding for endpoint.

Once we have the basic plumbing in place will create query to update row in users table, etc.

Test Plan: Will be tested implicitly by next diff which will try hitting endpoint from client.

Reviewers: ginsu, ashoat

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7108