Add support for Harmonic/Humble pairing

[azeey]

Fixes #461

Summary

Cherry-picks #447 and #448, update headers (3853cf2) and namespaces ( 66b61f2) and makes some tweaks to CMake and package.xml so that the humble branch can be built with Harmonic as well as Fortress.

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.

[bperseghetti]

Looks good, I would maybe suggest adding edits to the README.md though for harmonic as it's currently not listing Garden and hopefully soon Harmonic for Humble.

ROS 2 version	Gazebo version	Branch	Binaries hosted at
Foxy	Citadel	foxy	https://packages.ros.org
Foxy	Edifice	foxy	only from source
Galactic	Edifice	galactic	https://packages.ros.org
Galactic	Fortress	galactic	only from source
Humble	Fortress	humble	https://packages.ros.org
Humble	Garden	humble	gazebo packages
Humble	Harmonic	humble	gazebo packages
Rolling	Edifice	ros2	only from source
Rolling	Fortress	ros2	https://packages.ros.org
Rolling	Garden	ros2	only from source

[bperseghetti]

Looks good, I would maybe suggest adding edits to the README.md though for harmonic as it's currently not listing Garden and hopefully soon Harmonic for Humble.

ROS 2 version Gazebo version Branch Binaries hosted at
Foxy Citadel foxy https://packages.ros.org
Foxy Edifice foxy only from source
Galactic Edifice galactic https://packages.ros.org
Galactic Fortress galactic only from source
Humble Fortress humble https://packages.ros.org
Humble Garden humble gazebo packages
Humble Harmonic humble gazebo packages
Rolling Edifice ros2 only from source
Rolling Fortress ros2 https://packages.ros.org
Rolling Garden ros2 only from source

I should also mention that's what I think the README.md should look like.

Basically adding/editing:

Humble | Garden | [humble](https://github.com/gazebosim/ros_gz/tree/humble) | [gazebo packages](http://packages.osrfoundation.org/gazebo)
Humble | Harmonic | [humble](https://github.com/gazebosim/ros_gz/tree/humble) | [gazebo packages](http://packages.osrfoundation.org/gazebo)

Forgot I don't have approval permissions 🤣 well, there's my mostly non-meaningful review/approval.

[azeey]

Looks good, I would maybe suggest adding edits to the README.md though for harmonic as it's currently not listing Garden and hopefully soon Harmonic for Humble.

ROS 2 version Gazebo version Branch Binaries hosted at
Foxy Citadel foxy https://packages.ros.org
Foxy Edifice foxy only from source
Galactic Edifice galactic https://packages.ros.org
Galactic Fortress galactic only from source
Humble Fortress humble https://packages.ros.org
Humble Garden humble gazebo packages
Humble Harmonic humble gazebo packages
Rolling Edifice ros2 only from source
Rolling Fortress ros2 https://packages.ros.org
Rolling Garden ros2 only from source

I've updated the readme with two modifications from your suggestion:

• Link to https://gazebosim.org/docs/latest/ros_installation instead of package.osrfoundation.org
• Use "only from source" for Harmonic/Humble since that won't be available when this PR is merged. We'll update it when the binaries are released.