[SpaceCore] Add no-armour MonsterAdditionalData option to rock crabs #591

False Magma Caps are Rock Crabs under the hood (see RockCrab.cs), but are constantly vulnerable to damage rather than being armoured when not moving.

Adding an MonsterAdditionalData option for Rock Crabs to match this behaviour (like how armour being impossible to break can be toggled with IsStickBug) would allow spawning false mushrooms that behave as expected for a player.

Added after our conversation on Discord- it's just a nice-to-have (I could be so mean to players on mushroom levels!).