[identity] Add HTTP function to verify CSAT

Summary:
Added function that verifies Authorization header for valid CSAT.
The header format is compliant with what we have for HTTP services (blob, backup): Authorization: Bearer [base64-encoded JSON] - I could reuse some utils from comm-lib to easily achieve that.

Depends on D12302

Test Plan:
Created a dummy endpoint that calls verify_csat()? and returns HTTP 200. Verified the following:

• Missing Authorization header returns HTTP 401 Unauthorized
• Invalid header format returns HTTP 400 Bad request
  • Missing "Bearer " prefix
  • Invalid base64
  • Invalid JSON - e.g. missing one of required fields
• Invalid credentials return HTTP 403 Forbidden
• HTTP 200 (func returns Ok(())) for valid credentials taken from identity-tokens table

Reviewers: varun, will, marcin

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12303