Added descriptions to gz_server and ros_gz_sim launch files (backport #838)

[mergify]

🎉 New feature

Summary

I've added descriptions to args in gz_server.launch(|.py) and ros_gz_sim.launch(|.py).

I've also added propagation of initial_sim_time (was forgotten when it was added) and verbosity_level (new parameter added yesterday).

Test it

ros2 launch ros_gz_sim ros_gz_sim.launch --show-args
ros2 launch ros_gz_sim ros_gz_sim.launch.py --show-args
ros2 launch ros_gz_sim gz_server.launch --show-args
ros2 launch ros_gz_sim gz_server.launch.py --show-args
ros2 launch ros_gz_sim gz_server.launch world_sdf_file:=empty.sdf verbosity_level:=1 initial_sim_time:=100.0
gz topic -e -t /clock  # expect values starting with 100.0 s

However, #803 makes it impossible to test actually running the ros_gz_sim.launch files as I get:

[ERROR] [launch]: Caught exception in launch (see debug for traceback): Caught multiple exceptions when trying to load file of format [py]:
 - TypeError: 'RosGzBridge' expected 'bridge_params' to be one of [<class 'list'>], but got '<launch.substitutions.launch_configuration.LaunchConfiguration object at 0x7cd237a54f20>' of type '<class 'launch.substitutions.launch_configuration.LaunchConfiguration'>'
 - InvalidFrontendLaunchFileError: The launch file may have a syntax error, or its format is unknown

I suggest removing bridge_params from the launch file completely as was suggested in some of the discussions around #664 .

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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Generated-by: Human

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This is an automatic backport of pull request #838 done by Mergify.