More natural Magic Boulder support

[Starstrider42]

With the addition of Magic Boulders to asteroid code in 1.1.3, any asteroid can potentially be bouldered. The cleanest way for Custom Asteroids to support this is to make magic boulders a separate asteroid class, then emulate the stock occurrence rate.

[Starstrider42]

This issue can be done for asteroids at any time, but for comets is blocked on KSP#27070, since it prevents me from having more than one comet part.