Enhance blackboard integration for built-in tasks

[Anantes]

Problem statement

I'm trying to follow Single Source of Truth architecture and I'd like to set up multiple parameters for LimboAI BT using a blackboard. My game has multiple NPCs with a parameter named "attack_cooldown", and with built-in BTWait I have to explicitly write the duration in editor. There are two problems with this:

1. The same value has to be setted up twice: in the agent node and the BT timer
2. It's complicated to change the value during runtime (like applying buffs and debuffs)

Proposed solution

It would be reasonable if built-in features like BTWait or BTCooldown could grab their duration var from blackboard

Alternatives

Currently I'm solving it with creating my own Timer-based actions, but for me it seems such a base feature that it deserves to be in the box:

class_name WaitWithVar
extends BTWait

@export var duration_var: StringName = &"duration"


func _setup() -> void:
	self.duration = self.blackboard.get_var(duration_var)

[limbonaut]

Now that we have support for BBParam inspector editor in GDExtension too, I believe this should be unblocked. The trick is to implement these new properties while keeping backwards compatibility, so not to mess up existing user BT resources. This probably means we need some compatibility layer that would allow accepting deprecated old values from tres and convert them into new values on the fly.