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