[comm-lib] Let BlobServiceClient accept service-to-service token

Summary:
Updated BlobServiceClient to accept service-to-service token requests. This means HTTP services that rely on this Blob client, can accept service-to-service token and pass it through to Blob Service.

In short, previously:

• client -[CSAT]-> backup -[S2SToken]-> blob was possible
• identity -[S2SToken]-> backup -[S2SToken]-> blob wasn't possible

This diff makes the latter possible too, by overriding the accepts_services_token() for BlobServiceClient.

Test Plan: Ran Backup and Blob locally, supplied Backup request with service-to-service token instead of CSAT, made sure the COMM_SERVICES_DISABLE_CSAT_VERIFICATION is disabled. Request succeeded.

Reviewers: kamil, will, varun

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13318