[keyserver] introduce create_or_update_farcaster_channel_tag endpoint

Summary:
This diff introduces the create_or_update_farcaster_channel_tag endpoint so that we can call the createOrUpdateFarcasterChannelTag function we just introduced from the client

Depends on D11808

Test Plan: In my local stack I was able to call the create_or_update_farcaster_channel_tag endpoint and use createOrUpdateFarcasterChannelTag function from the client

Reviewers: bartek, atul, tomek

Reviewed By: tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11773