[lib] use fc channel avatar url in useResolvedThreadAvatar

Summary:
Depends on D13743

useResolvedThreadAvatar can take an object containing a username and FID or an object containing a farcaster channel ID.

the former is when we want to display a user avatar as the thread avatar

we want these types to be mutually exclusive to avoid confusion when the avatar type is 'farcaster' and the function caller provides a farcaster channel ID and a user FID

Test Plan: tested in next diffs by successfully rendering farcaster channel images as thread avatars

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13744