feat: Add HoverCallbacks (#2706)

This creates HoverCallbacks (and PointerMoveCallbacks) to replicate the Hoverables behaviour in the new camera and event system.

examples/lib/stories/input/hover_callbacks_example.dart

@@ -0,0 +1,40 @@
+import 'package:flame/components.dart';
+import 'package:flame/events.dart';
+import 'package:flame/extensions.dart';
+import 'package:flame/game.dart';
+import 'package:flutter/material.dart';
+
+class HoverCallbacksExample extends FlameGame with TapCallbacks {
+  static const String description = '''
+    This example shows how to use `HoverCallbacks`s.\n\n
+    Add more squares by clicking and hover them to change their color.
+  ''';
+
+  @override
+  Future<void> onLoad() async {
+    add(HoverSquare(Vector2(200, 500)));
+    add(HoverSquare(Vector2(700, 300)));
+  }
+
+  @override
+  void onTapDown(TapDownEvent event) {
+    add(HoverSquare(event.localPosition));
+  }
+}
+
+class HoverSquare extends PositionComponent with HoverCallbacks {
+  static final Paint _white = Paint()..color = const Color(0xFFFFFFFF);
+  static final Paint _grey = Paint()..color = const Color(0xFFA5A5A5);
+
+  HoverSquare(Vector2 position)
+      : super(
+          position: position,
+          size: Vector2.all(100),
+          anchor: Anchor.center,
+        );
+
+  @override
+  void render(Canvas canvas) {
+    canvas.drawRect(size.toRect(), isHovered ? _grey : _white);
+  }
+}