feat(*): Add self-play RL training pipeline with PPO trainer, in-game GDScript policy inference, and bot opponent support in Match mode

Game/addons/godot_rl_agents/rewards/ApproachNodeReward2D.gd

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+extends RewardFunction2D
+class_name ApproachNodeReward2D
+
+## Calculates the reward for approaching node
+## a reward is only added when the agent reaches a new
+## best distance to the target object.
+
+## Best distance reward will be calculated for this object
+@export var target_node: Node2D
+
+## Scales the reward, 1.0 means the reward is equal to 
+## how much closer the agent is than the previous best.
+@export_range(0.0, 1.0, 0.0001, "or_greater") var reward_scale: float = 1.0
+
+var _best_distance
+
+
+func get_reward() -> float:
+	var reward := 0.0
+	var current_distance := global_position.distance_to(target_node.global_position)
+	if not _best_distance:
+		_best_distance = current_distance
+	if current_distance < _best_distance:
+		reward = (_best_distance - current_distance) * reward_scale
+		_best_distance = current_distance
+	return reward
+
+
+func reset():
+	_best_distance = null