[lib] Use identity service for keyserver session recovery

Summary:
This diff is gated on usingCommServicesAccessToken. We fall back to legacy keyserver auth if the new identity-based workflow doesn't work.

Depends on D11499

Test Plan:
Tested in combination with later diffs:

1. Set up multi-keyserver environment following Notion doc
2. Test native keyserver recovery via keyserverAuth by deleting a cookie in MariaDB console
3. Hardcode incorrect userID, try same test as 2, and confirm that it fails
4. Test web keyserver recovery in the same way as in 2
5. Test incorrect userID on web in the same way as in 3

Reviewers: tomek, inka

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D11500