[lib] Expose callKeyserverEndpoint in CallKeyserverEndpointContext

Summary:
This will be used in the next diff, where we'll need to construct actions manually in KeyserverConnectionHandler so that it can work in a recovery and non-recovery context.

Depends on D11497

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/D11498