[comm-lib] Add remove_multiple_holders to Blob client

Summary:
We're missing functions for batch holders endpoints in BlobServiceClient.
Started with the one that's needed for server-side holder cleanup

Depends on D13617

Test Plan:

1. Started Blob service. Created a few holders.
2. Created a dummy test that calls this function.
3. Ran cargo test and verified that these holders were now removed from DDB

Reviewers: kamil, varun, will

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13644