Add is_instance_id_valid() method to allow GDNative ptr caching

Created by: Cygon

This adds a method is_instance_id_valid() to the GDNative API that language bindings can use to detect when stored node pointers become invalid.

This achieves a small performance gain because the alternative is to store only the NodePath and resolve (as well as cast) the node from the scene each time it needs to be accessed:

Without this is_instance_id_valid():

// Needs to follow a NodePath each frame and repeat the dynamic cast
godot::KinematicBody *get_kinematic_body_node() const {
    const godot::Node *target = get_node_or_null(this->kinematic_body_node_path);
    if(target == nullptr) {
        return nullptr;
    } else {
        return godot::Object::cast_to<ComponentType>(target);
    }
}

With is_instance_id_valid():

// Can reuse the pointer if the (also stored) id still checks out
godot::KinematicBody *get_kinematic_body_node() const {
    if(this->kinematic_body_node != nullptr) {
        if(godot::Object::is_instance_id_valid(this->kinematic_body_instance_id)) {
            return this->kinematic_body_node;
        }
    }

    // Stored pointer was null or id didn't check out
    const godot::Node *node = get_node_or_null(this->kinematic_body_node_path);
    if(node == nullptr) {
        this->kinematic_body_node = nullptr;
    } else {
        this->kinematic_body_node = godot::Object::cast_to<godot::KinematicBody>(node);
        if(this->kinematic_body_node != nullptr) {
            this->kinematic_body_instance_id = this->kinematic_body_node->get_instance_id();
        }
    }

    return this->kinematic_body_node;
}

This implements the changes proposed in #28478 (closed).

Note that I added a call to rw_lock->read_lock() to both instance_id_validate() and instance_validate() as this seems to be the way all methods inside core/object.cpp handle things (but perhaps the future could be a lock-free hash table if an implementation exists that works on all required platforms)