[lib][web][native] Send CSAT in getKeyserverOverrideForAnInviteLink

Summary:
getKeyserverOverrideForAnInviteLink() performs a Blob Service request but doesn't send Authorization header.
Followed multimedia fetching code and added optional AuthMetadata that is sent if usingCSAT == true.

Test Plan:

• On my local keyserver, created a community and created invite link
• Opened the link in the browser - invitation was successful.
• Searched Blob Service debug logs - Authorization header was present.

Reviewers: tomek

Reviewed By: tomek

Subscribers: ginsu, ashoat

Differential Revision: https://phab.comm.dev/D12437