[lib] Initial NeynarClient with fetchFriendFIDs method

Summary:
This class will be used for querying Neynar for the FIDs of the "friends" (mutual follows) of the user.

Depends on D11387

Test Plan: Tested this using my own FID by hooking it into FarcasterDataHandler in a later commit

Reviewers: atul, ginsu

Reviewed By: ginsu

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11388