control usage validators can produce warning + validate attached properties

[exyi]

It's not possible to add compilation warnings using control usage validators. It is also possible to validate attached properties on foreign controls.

        [ControlUsageValidator(IncludeAttachedProperties = true)]
        public static IEnumerable<ControlUsageError> ValidateUsage(ResolvedControl control)
        {
               if (control.TryGetProperty(MyAttachedProperty, out var prop))
                    yield return new ControlUsageError("Test warning", DiagnosticSeverity.Warning, prop.DothtmlNode);
        }

Used to implemented check that Validation.Target is not a primitive type (see #1583)