Add covariants to reduce subclass casts in 2D APIs (flutter#141318)

While working in https://pub.dev/packages/two_dimensional_scrollables, I found I could eliminate some casts if we added covariants here in the framework.
Much of the 2D aPI in the framework is abstract, so I think it make sense to add these and make it easier/cleaner for subclasses using the APIs.
I made a similar change in flutter#131358, this would cover all of the cases I could find so its nice and accommodating now.

packages/flutter/lib/src/widgets/scroll_delegate.dart

@@ -895,7 +895,7 @@ abstract class TwoDimensionalChildDelegate extends ChangeNotifier {
   /// widgets have changed, a new delegate must be provided, and the new
   /// delegate's [shouldRebuild] method must return true. Alternatively,
   /// calling [notifyListeners] will allow the same delegate to be used.
-  Widget? build(BuildContext context, ChildVicinity vicinity);
+  Widget? build(BuildContext context, covariant ChildVicinity vicinity);
 
   /// Called whenever a new instance of the child delegate class is
   /// provided.

packages/flutter/lib/src/widgets/two_dimensional_viewport.dart

@@ -793,7 +793,7 @@ abstract class RenderTwoDimensionalViewport extends RenderBox implements RenderA
   ///
   /// Returns null if there is no active child for the given [ChildVicinity].
   @protected
-  RenderBox? getChildFor(ChildVicinity vicinity) => _children[vicinity];
+  RenderBox? getChildFor(covariant ChildVicinity vicinity) => _children[vicinity];
 
   @override
   void attach(PipelineOwner owner) {

packages/flutter/test/widgets/two_dimensional_viewport_test.dart

@@ -2714,6 +2714,38 @@ void main() {
   });
 }
 
+class _TestVicinity extends ChildVicinity {
+  const _TestVicinity({required super.xIndex, required super.yIndex});
+}
+
+class _TestBaseDelegate extends TwoDimensionalChildDelegate { //ignore: unused_element
+  // Would fail analysis without covariant
+  @override
+  Widget? build(BuildContext context, _TestVicinity vicinity) => null;
+
+  @override
+  bool shouldRebuild(covariant TwoDimensionalChildDelegate oldDelegate) => false;
+
+}
+
+class _TestBuilderDelegate extends TwoDimensionalChildBuilderDelegate { //ignore: unused_element
+  _TestBuilderDelegate({required super.builder});
+  // Would fail analysis without covariant
+  @override
+  Widget? build(BuildContext context, _TestVicinity vicinity) {
+    return super.build(context, vicinity);
+  }
+}
+
+class _TestListDelegate extends TwoDimensionalChildListDelegate { //ignore: unused_element
+  _TestListDelegate({required super.children});
+  // Would fail analysis without covariant
+  @override
+  Widget? build(BuildContext context, _TestVicinity vicinity) {
+    return super.build(context, vicinity);
+  }
+}
+
 RenderTwoDimensionalViewport getViewport(WidgetTester tester, Key childKey) {
   return RenderAbstractViewport.of(
     tester.renderObject(find.byKey(childKey))
@@ -2779,6 +2811,11 @@ class _SomeRenderTwoDimensionalViewport extends RenderTwoDimensionalViewport { /
     super.delegate = value;
   }
 
+  @override
+  RenderBox? getChildFor(_TestVicinity vicinity) { // Analysis would fail without covariant
+    return super.getChildFor(vicinity);
+  }
+
   @override
   void layoutChildSequence() {}
 }