feat: The FunctionEffect, run any function as an Effect (#3537)

The `FunctionEffect` is super simple, but very powerful.
It makes it possible to run any function over time as an effect without
having to create a new effect.
This is very useful when for example doing state changes over time.

examples/lib/stories/effects/function_effect_example.dart

@@ -0,0 +1,64 @@
+import 'package:flame/components.dart';
+import 'package:flame/effects.dart';
+import 'package:flame/game.dart';
+import 'package:flame/input.dart';
+
+enum RobotState {
+  idle,
+  running,
+}
+
+class FunctionEffectExample extends FlameGame with TapDetector {
+  static const String description = '''
+This example shows how to use the FunctionEffect to create custom effects.
+
+The robot will switch between running and idle animations over the duration of
+10 seconds.
+''';
+
+  @override
+  Future<void> onLoad() async {
+    final running = await loadSpriteAnimation(
+      'animations/robot.png',
+      SpriteAnimationData.sequenced(
+        amount: 8,
+        stepTime: 0.2,
+        textureSize: Vector2(16, 18),
+      ),
+    );
+    final idle = await loadSpriteAnimation(
+      'animations/robot-idle.png',
+      SpriteAnimationData.sequenced(
+        amount: 4,
+        stepTime: 0.4,
+        textureSize: Vector2(16, 18),
+      ),
+    );
+    final robotSize = Vector2(64, 72);
+
+    final functionEffect =
+        FunctionEffect<SpriteAnimationGroupComponent<RobotState>>(
+      (target, progress) {
+        if (progress > 0.7) {
+          target.current = RobotState.idle;
+        } else if (progress > 0.3) {
+          target.current = RobotState.running;
+        }
+      },
+      EffectController(duration: 10.0, infinite: true),
+    );
+    final component = SpriteAnimationGroupComponent<RobotState>(
+      animations: {
+        RobotState.running: running,
+        RobotState.idle: idle,
+      },
+      current: RobotState.idle,
+      position: size / 2,
+      anchor: Anchor.center,
+      size: robotSize,
+      children: [functionEffect],
+    );
+
+    add(component);
+  }
+}