new Ignition Gazebo simulation interface architecture (#20057)

- much simpler direct interface using Ignition Transport 
 - in tree models and worlds
 - control allocation output configuration, no more magic actuator mapping to mavlink and back
 - currently requires no custom Gazebo plugins (keeping things as lightweight and simple as possible)

Co-authored-by: Jaeyoung-Lim <jalim@ethz.ch>

platforms/posix/Debug/launch_sim.json.in

@@ -145,18 +145,10 @@
             "id": "PX4_SIM_MODEL",
             "description": "PX4_SIM_MODEL",
             "options": [
-              "iris",
-              "typhoon_h480",
-              "plane",
-              "plane_catapult",
-              "plane_lidar",
-              "standard_vtol",
-              "tailsitter",
-              "tiltrotor",
-              "r1_rover",
-              "boat"
+              "x3",
+              "x4"
             ],
-            "default": "iris"
+            "default": "x3"
           }
     ]
 }