[native] Send CSAT when fetching blob service media

Summary:
fetchAndDecryptMedia performs a Blob Service request but doesn't send Authorization header
unless authMetadata is provided.
Updated the callsite to provide it when usingCommServicesAccessToken = true

Test Plan:
Tested locally with CSAT verification enabled for services.
Blob-hosted encrypted media are downloaded and rendered correctly.

Reviewers: kamil, tomek

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12446