[lib] add getFCAvatarURLs method to NeynarClientProvider

Summary:
Based this off the existing client provider method for FC usernames

Deduping made this code harder to follow. That said, I can revise this if reviewers think it would be better anyway

Depends on D13098

Test Plan: tested at end of stack by successfully fetching farcaster avatar URL with neynar

Reviewers: ashoat, tomek

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13099