initial minimal PX4_ROS2 platform and px4_ros2_default build (#20689)

- new ROS2 platform in PX4 intended for creating configs that build and run entirely in ROS2
 - PX4_CONFIG defaults to px4_ros2_default if no config specified and in a colcon workspace with ROS_VERSION=2
 - currently doesn't do much other than allow you to build px4 msgs interface package

platforms/ros2/include/px4_platform_common/Node.hpp

@@ -0,0 +1,42 @@
+
+#pragma once
+
+#include "rclcpp/rclcpp.hpp"
+
+namespace px4
+{
+
+class Node : public rclcpp::Node
+{
+public:
+	explicit Node(const char *node_name) :
+		rclcpp::Node(node_name)
+	{
+
+	}
+
+
+	~Node() override = default;
+
+private:
+
+};
+
+};
+
+
+// int main(int argc, char *argv[])
+// {
+// 	rclcpp::init(argc, argv);
+// 	rclcpp::spin(std::make_shared<MulticopterRateControl>());
+// 	rclcpp::shutdown();
+// 	return 0;
+// }
+
+
+// #include "rclcpp_components/register_node_macro.hpp"
+
+// // Register the component with class_loader.
+// // This acts as a sort of entry point, allowing the component to be discoverable when its library
+// // is being loaded into a running process.
+// RCLCPP_COMPONENTS_REGISTER_NODE(composition::Client)