[2.x] Add Custom Casts Classifier from Laravel 7.0

[stefanzweifel]

Laravel 7.0 will ship with several new features. One of them are Custom Casts.

I see them as new core component, so it makes sense to update stats to recognise them as such.

Custom Casts are easily recognisable, as they implement the Illuminate\Contracts\Database\Eloquent\CastsAttributes or Illuminate\Contracts\Database\Eloquent\CastsInboundAttributes interfaces.

A new classifier should look very similar to the existing RuleClassifier.

I'm happy to accept a well tested PR for this.