Support physical worlds when launching robots

[aquintan4]

Physical worlds could not be launched through RAM.

• LauncherRobot had an empty physical entry in worlds, so a robot in a
  scene of type physical raised a KeyError. It now uses the same
  robot_ros2_api launcher as gz, which runs ros2 launch on the robot
  launch file without spawning anything in the simulator.
• LauncherRos2Api, used by physical scenes, defined wait_robot_spawn while
  the scene launcher calls wait_robots_spawn, so launching a physical world
  raised an AttributeError after the robot had started. Renamed to match.

Tested with a physical MiR100 world in a local RADI, the robot launches and
its topics are available. No change for existing gz worlds.