Allow setting custom gauges as message gauges

[MjnMixael]

Custom scripted messages gauges are essentially standard now, so we need a way to tell FSO that this gauge should stay enabled when hud-disabled-except-messages is called. So this adds a boolean table option to set a custom gauge as a message gauge which were previously always hardcoded to false.

Fixes #7197

[naomimyselfandi]

The existing code is BUTTS, but this is probably the most straightforward way to hook into it. In the interest of practicing what I preach, could you confirm that you've tested it before I approve it?

[MjnMixael]

The existing code is BUTTS, but this is probably the most straightforward way to hook into it. In the interest of due diligence, could you confirm that you've tested it independently?

Yup. I tested it on our gauges in BtA and it worked as expected.