Add service success setting in ROS2SpawnerComponent::GetAvailableSpawnableNames

[knicked]

What does this PR do?

The /get_available_spawnable_names service in ROS2 Spawner always returned success=False in response, while there were no issues in calling the service. This PR adds setting the success field to true in the appropriate method.

How was this PR tested?

A scene with ROS2 Spawner was loaded and played in the editor and the service was called:

$ ros2 service call /get_available_spawnable_names gazebo_msgs/srv/GetWorldProperties
requester: making request: gazebo_msgs.srv.GetWorldProperties_Request()

response:
gazebo_msgs.srv.GetWorldProperties_Response(sim_time=0.0, model_names=['tomato', 'green_cube', 'apple', 'blue_cube', 'corn', 'toy_box', 'carrot', 'red_cube', 'yellow_cube'], rendering_enabled=False, success=True, status_message='')