BTPlayer: Allow switching BT instance at runtime

[limbonaut]

Usage example:

@export var behavior_tree: BehaviorTree

var instance: BTInstance

@onready var bt_player: BTPlayer = $BTPlayer

func _ready() -> void:
    # Initialize behavior tree instance to be used later
    var agent: Node = self
    var scene_root: Node = bt_player.get_owner()
    var blackboard: Blackboard = behavior_tree.blackboard_plan.create_blackboard(scene_root)
    instance = behavior_tree.instantiate(agent, blackboard, bt_player)

...

func _on_some_event():
    # Switch behavior tree instance upon some event
    bt_player.set_bt_instance(instance)

Note: Code written from memory - might contain errors.