[lib] introduce fetchFollowedFarcasterChannels to neynar client class

Summary:
fetchFollowedFarcasterChannels is a function that given a user fid will retrieve all channels that the user follows on farcaster

Endpoint reference in neynar docs: https://docs.neynar.com/reference/user-channels

Linear task: https://linear.app/comm/issue/ENG-7465/add-the-user-channels-neynar-endpoint-to-our-neynar-api-client

Depends on D11675

Test Plan:
Passed in my personal fid and confirmed that the results I got from this function match exactly what is shown on warpcast

my channels I follow on warpcast:

results of fetchFollowedFarcasterChannels (intentionally only logged the channel name to make it easy to view all the results):

Reviewers: atul, inka, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11676