API access with client credentials (core functionality) (#16817)

* First stab

* Delivery API client credentials + a little refactor to ensure unique client IDs

* Introduce user type

* Support user type in the Management API

* Clean up TODOs

* Update API user last login date when issuing a token

* Better error reporting for mismatched user types

* Do not allow password change or reset for API users

* Update OpenApi.json

* Revert change

* Remove obsolete comment

* Make applicable classes abstract or sealed

* Review changes

* Add endpoint for retrieving all user client IDs

src/Umbraco.Infrastructure/Security/BackOfficeUserStore.cs

@@ -141,6 +141,7 @@ public class BackOfficeUserStore :
             StartMediaIds = user.StartMediaIds ?? new int[] { },
             IsLockedOut = user.IsLockedOut,
             Key = user.Key,
+            Type = user.Type
         };