Add required modifiers and generalize behaviour type

Marked UrlLocation properties as required in ResourceDefinition for improved null safety. Changed OrderedLinkGenerator to use the more general IStateChangeBehaviour instead of NumberedStateChanger, increasing flexibility.

Beam.Models/ResourceDefinition.cs

@@ -19,8 +19,8 @@ public class ResourceDefinition {
     }
 
     public class UrlLocation {
-        public ILinkBuilder Segments { get; init; }
-        public List<ImmutableState> States { get; init; }
-        public IStateChangerFactory StateChanger { get; init; }
+        public required ILinkBuilder Segments { get; init; }
+        public required List<ImmutableState> States { get; init; }
+        public required IStateChangerFactory StateChanger { get; init; }
     }
 }