Add a-star based global planner

mg_planner/src/mg_planner.cpp

@@ -0,0 +1,11 @@
+#include "rclcpp/rclcpp.hpp"
+
+#include "mg_planner/mg_planner_node.hpp"
+#include <rclcpp/executors.hpp>
+
+int main(int argc, char** argv) {
+    rclcpp::init(argc, argv);
+    rclcpp::spin(std::make_shared<mg::PlannerNode>());
+
+    return 0;
+}