Add filter/query for domain on /pages/domains API endpoint

Proposal

Add a filter/query for domain on the list all pages domain API endpoint to allow for an instance adminstrator to search for a specific domain. This would allow administrators to easily identify whether a domain is verified or not.

The desired outcome would be to be able to make the below call to get only the record showing which project has verified the gitlab.com domain in this case:

GET https://gitlab.com/api/v4/pages/domains?domain=gitlab.com

Within the GitLab Support team, this functionality would be useful for many scenarios, including changing the email address of an enterprise user. In this case, it would be beneficial for the support team to verify whether the domain is verified elsewhere before making such a change.

Without knowing whether the domain is verified elsewhere, it is possible that the users could be claimed as enterprise users of another namespace and potentially lose access to their account if the enterprise group disabled authentication methods for enterprise users.