Added terrain surface data notifications (#5067)

* Fix notifications for surface data changes.
Separated the notifications from the surface component and the height component to add a reason to a RefreshArea request.  This makes it possible to distinguish between surface changes and height changes and provide the appropriate OnTerrainDataChanged flags.

Signed-off-by: Mike Balfour <82224783+mbalfour-amzn@users.noreply.github.com>

* Reworked to use a changeMask instead of separate calls.

Signed-off-by: Mike Balfour <82224783+mbalfour-amzn@users.noreply.github.com>

* PR Feedback
Judicious use of "using" to reduce a bunch of bulky namespaces.

Signed-off-by: Mike Balfour <82224783+mbalfour-amzn@users.noreply.github.com>

Gems/Terrain/Code/Source/Components/TerrainSurfaceGradientListComponent.cpp

@@ -184,7 +184,9 @@ namespace Terrain
 
     void TerrainSurfaceGradientListComponent::OnCompositionChanged()
     {
-        TerrainSystemServiceRequestBus::Broadcast(&TerrainSystemServiceRequestBus::Events::RefreshArea, GetEntityId());
+        TerrainSystemServiceRequestBus::Broadcast(
+            &TerrainSystemServiceRequestBus::Events::RefreshArea, GetEntityId(),
+            AzFramework::Terrain::TerrainDataNotifications::SurfaceData);
     }
 
 } // namespace Terrain