[comm-lib][blob] Add tags support to remove holders request payload

Summary:
Expanded RemoveHoldersRequest to support either current payload format, or the new one that
removes holders that matches given tags. The new format is not supported yet, so added HTTP 501 Unimplemented response server side.

Depends on D13645

Test Plan: Unit tests for serialize/deserialize to make sure that existing request format is untouched.

Reviewers: kamil, varun, will

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13646