[lib] implement fetchFarcasterCastByHash for neynar client

Summary:
This implements the fetchFarcasterCastByHash function for the neynar client

Depends on D13756

Test Plan:
Tested by running on my keyserver and plugging in a cast hash. Successfully console logged response

Reviewers: varun, ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13762