[lib] update getAvatarForUser signature

Summary: getAvatarForUser just needs a type that contains optional, nullable username and avatar fields. modifying the function signature to make it clear what data we're dealing with here.

Test Plan: tested with rest of stack by successfully setting thread and user avatars

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13450