Clarify the HostStatus service in name and purpose

Problem/Motivation

The class name and comments for \Drupal\disable_login_by_domain\HostStatus are confusing. It is not clear what exactly this class does.

Proposed resolution

- Add an interface for this class, including clear usage and purposes.
- Make this class implement this interface.
- Consider renaming the class from HostStatus to something like DisableDomains or something similar.

Remaining tasks

- Do the above

API changes

- Class name may change