[lib] Introduce ClientAvatar and introduce avatar field for *UserInfo types

Summary: Add avatar field to *UserInfo types. Based on discussion w/ @ginsu.

Test Plan: Will be tested implicitly by subsequent diffs.

Reviewers: ginsu, ashoat

Reviewed By: ginsu, ashoat

Subscribers: tomek, ginsu

Differential Revision: https://phab.comm.dev/D7109