Add post and delete methods to user-group/id/users to add/remove user… (#15490)

* Add post and delete methods to user-group/id/users to add/remove users from group

* Update OpenApi contract

* PR feedbac

Moved fetch usergroup logic into service layer
Renamed methods to async

* Naming

* Introduced user not found on UserGroupOperationStatus, as otherwise the return message is wrong

* Added authentication

* Removed authorization from the service layer as its determined that that responsibiliity does not belong there.

---------

Co-authored-by: Sven Geusens <sge@umbraco.dk>
Co-authored-by: Bjarke Berg <mail@bergmania.dk>

src/Umbraco.Core/Models/UsersToUserGroupManipulationModel.cs

@@ -0,0 +1,15 @@
+using Umbraco.Cms.Core.Models.Membership;
+
+namespace Umbraco.Cms.Core.Models;
+
+public class UsersToUserGroupManipulationModel
+{
+    public Guid UserGroupKey { get; init; }
+    public Guid[] UserKeys { get; init; }
+
+    public UsersToUserGroupManipulationModel(Guid userGroupKey, Guid[] userKeys)
+    {
+        UserGroupKey = userGroupKey;
+        UserKeys = userKeys;
+    }
+}