We should consider allowing an admin hostname per realm, rather than just supporting a single hostname-admin.

There are two implemenation approaches:

1. Have the hostname provider determine the admin URL for a given realm based upon the security-admin-console client root URL. This would be compatible with anyone currently using hostname-strict=false along with customized values for their security-admin-console client root URLs.
2. Introduce a realm setting, similar to the realm specific Frontend URL, to set the admin URL, which will then be used by the hostname provider.

The admin hostname could be used by the hostname check mentioned in #47453.

cc @vmuzikar