[lib] rename useFarcasterAvatarURL -> useFarcasterUserAvatarURL

Summary:
Depends on D13722

just a rename before I introduce a useFarcasterChannelAvatarURL hook later in stack

Test Plan: flow

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13723