[lib] change fetchFarcasterChannelByName -> fetchFarcasterChannelByID and refactor

Summary:
realized we're conflating channel name and ID in our code. this change cleans things up a little and makes sure we're querying by ID

Depends on D13695

Test Plan: called fetchFarcasterChannelByID with a channel name ("Crypto Fantasy League") and didn't get back a response. called it again with a channel ID ("fantasyisland") and got the right response

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13718