[lib] add hook to fetch channel avatar url

Summary:
Depends on D13742

we need this hook to get the farcaster channel image URL in our useResolvedThreadAvatar hook

Test Plan: was able to successfully fetch URL from useResolvedThreadAvatar and display a farcaster channel image as a thread avatar

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13743