Add snap packaging

[artivis]

This PR adds snap packaging to rosboard.

To give it a shot, first install it:

snap install rosboard --channel=edge

And off you go:

rosboard

You don't even need ROS to be installed on the host to use it since the snap packages all rosboard dependencies.

At the moment I own the 'rosboard' name on the snap store; I'd be more than happy to transfer it to you.

Please, feel free to ping me with any question you might have.

A few notes:

• The snap currently on the store was packaged based on Noetic. Altho I tested it fine against my Melodic simulation.
• I've added every dependency that explicitly appears in the codebase - whether they are optional or not - to the package.xml file so that e.g. rosdep can install them.
• Snap being confined, it cannot access ROS packages on the host machine. Therefore you need to bundle every message that you might want to support. For instance, I've added a couple more typical ROS msgs here. One could grow that list based on user requests for instance.
• The dmesg visualizer is 'disabled' at the moment

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Marked as draft until canonical/snapcraft#3570 gets merged.
Marking the PR as ready since canonical/snapcraft#3570 is merged and released on the 'edge' channel.