Also extends AddAttributeAsync with an optional dataSourceReference parameter
so the fixture attribute appears in both _bindingDataSourceAttrs (bindings UI)
and _overrideAttrs (overrides UI) on the InstanceConfigure page.
In Disable_Instance_ShowsOutcomeToast and Enable_Instance_ShowsOutcomeToast, the
precondition CLI calls (DeployInstanceAsync / DisableInstanceAsync) were between
CreateInstanceAsync and the try block. A throw there would skip the finally
DeleteInstanceAsync, leaking the instance. Moved those calls to be the first
statements inside try so cleanup always runs once the instance exists.
Joseph Doherty