Add an action node that make a call to an existing function

[Earewien]

Request:

Create a behavior tree node that can make a direct call to an existing function instead of creating a new script to create the function.

The aim is always the same, to simplify developper's life, by avoiding to create new script for every specific action. Instead, he can reference an existing function in its GDScript. It allow to contains all logic into a single file instead of creating multiple files every where.