[native] Auth with authoritative keyserver directly from registration flow

Summary:
In an earlier diff, we extracted useKeyserverAuth out of KeyserverConnectionHandler. In this diff, we call it directly from the registration flow.

This addresses ENG-7669.

Depends on D11762

Test Plan:
This whole stack was tested with the following steps:

1. On native, attempting to log in with an ETH account that hasn’t been registered yet
2. On native, attemping to register with an ETH account that has already been registered
3. On native, log in with an ETH account that has already been registered
4. On native, register with an ETH account that hasn’t already been registered
5. On native, register with a password account
6. On native, log in with a password account
7. On web, attempting to log in with an ETH account that hasn’t been registered yet
8. On web, log in with an ETH account that has already been registered
9. On web, log in with a password account

Reviewers: tomek, inka

Reviewed By: inka

Differential Revision: https://phab.comm.dev/D11763