[keyserver/lib] introduce delete_farcaster_channel_tag endpoint

Summary:
This diff introduces the delete_farcaster_channel_tag endpoint so that we can call the deleteFarcasterChannelTag we just introduced

Depends on D11813

Test Plan: In my local stack I was able to call the delete_farcaster_channel_tag endpoint and use the deleteFarcasterChannelTag from the client

Reviewers: atul, bartek, tomek

Reviewed By: tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11814