[keyserver] introduce deleteFarcasterChannelTag

Summary:
This diff introduces the barebones deleteFarcasterChannelTag function. For the scope of this project we just want this function to delete the farcaster channel tag blob from the blob service for now, with the intention that all the other necessary parts for this function (like permissions, updating the row in the community table on the keyserver to remove the blob holder will be added to this function as a follow up project that I will start shortly)

Depends on D11809

Test Plan:
In my local stack I was able to use deleteFarcasterChannelTag to delete the farcaster channel => comm community blob.

I also confirmed that when the GATE_TAG_FARCASTER_CHANNEL gate is set to true the function throws a server error

Reviewers: atul, bartek, tomek

Reviewed By: tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11813