[lib] add new cache for fc channel info

Summary:
we will use this cache to fetch farcaster channel image URLs (and later, descriptions)

this is modeled after the fc cache for users

Test Plan: tested at end of stack, made sure that we're only calling neynar API once per channel ID

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13742