[blob] New endpoint to batch-delete holders

Summary:
Address ENG-4842.
Request and response format described in the Linear issue.

Created a new DELETE /holders endpoint for removing multiple holders in a single request.

Test Plan: Ran Blob locally. Called the endpoint using Postman and made sure holders are removed from DDB.

Reviewers: will, kamil, marcin, tomek

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13479