[keyserver] Add a function that deletes a blob

Summary:
This function calls an endpoint of Blob service that deletes a blob.

Depends on D9944

Test Plan: Create a blob and delete it - check if a holder is deleted.

Reviewers: michal, bartek, rohan

Reviewed By: bartek, rohan

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9966