Expose header for GzServer

[roncapat]

Closes #631

Summary

Currently, no header file is defined for https://github.com/gazebosim/ros_gz/blob/ros2/ros_gz_sim/src/gzserver.cpp

This means that currently there is no possibility to import the node in a custom main.cpp to be statically composed with other nodes in the same process space, but the user is forced to go with dynamic composition.

This PR exposes the class via its own header file.

CC @mjcarroll @azeey

Checklist

• Signed all commits for DCO
• Added tests
• 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.

[ahcorde]

I can see this error in CI

17:19:02 /tmp/ws/src/ros_gz/ros_gz_sim/src/gzserver.cpp:23:10: fatal error: ros_gz_sim/gzserver.hpp: No such file or directory
17:19:02    23 | #include <ros_gz_sim/gzserver.hpp>

[roncapat]

Fixed & rebased