Active Directory authenticator overrides the previously registered authenticator

[trrenty]

Steps to reproduce:

1. Have a xwiki instance with Integration Azure OAuth installed
2. Install Application Active Directory
3. Try to login using Azure
   Expected: Successful login
   Result: The login failed and you are redirected to Main without being logged

This is because this listener https://github.com/xwikisas/application-activedirectory/blob/master/application-activedirectory-api/src/main/java/com/xwiki/activedirectory/internal/ActiveDirectorySetupListener.java simply overrides the existing auth service.

Note that the same might happen in the case of Azure OAuth Integration. Since it registers the auth service in the same manner.

A possible solution would be for the auth service to fallback on the previously registered authenticator in case if it fails to login.