Add support for rare enemies

[SapiensAnatis]

Context
In the original game, rare enemies could spawn at random in levels. They would drop materials that were otherwise difficult to obtain.

The feature request
Dawnshard should spawn rare enemies. In particular, this makes it easier to access four-leafed clovers which are very difficult to come by at the moment, significantly slowing down dragon bond progress.

[SapiensAnatis]

Possible approaches:

• The enemy script could be reworked to specify a property against each param_id indicating whether it is rare. EnemyEncountParam assets in AssetStudio seem to have a rareEnemyGroup property set for rare enemies
• Could use Tough == 1 which indicates a rare enemy. Would not enable removing the workaround imposed in Fix chronos not spawning #516 because Chronos's Tough is 5