Launch gzserver from xml

[caguero]

🎉 New feature

Part of https://github.com/gazebosim/ros_gz/issues/552

Summary

This patch allows to run gzserver (as an executable or as a composable node) from an xml file.

How to test it?

Use the provided gzserver.launch to run gzserver as an executable:

ros2 launch ros_gz_sim gz_server.launch world_sdf_file:=empty.sdf

Confirm that you see gzserver loading and verify that no container has been created:

caguero@cold:~/ros_gz_ws$ ros2 component list

Now CTRL-C gzserver and try the composable version:

ros2 launch ros_gz_sim gzserver.launch world_sdf_file:=empty.sdf use_composition:=True

Confirm that you see gzserver loading and verify that a new container has been created:

caguero@cold:~/ros_gz_ws$ ros2 component list
/ros_gz_container
  1  /gzserver

Test it

Checklist

• [x] 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)
• [ ] While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[caguero]

@mjcarroll , is this the idea that you had in mind with gzserver.launch?