Add predicate property to MGLStyleLayer

[1ec5]

MGLStyleLayer should have a predicate property that translates to a filter in mbgl. The developer would create a predicate like so:

[NSPredicate predicateWithFormat:@"name == 'USA'"]

The returned value would be an instance of NSComparisonPredicate with leftExpression.keyPath being name, predicateOperatorType being NSEqualToPredicateOperatorType, and rightExpression.constantValue being USA. We’d translate that into an mbgl::style::EqualsFilter.

Here’s a full correspondence between NSPredicate types and style specification filter forms.

/cc @frederoni @incanus

[boundsj]

This was completed by @frederoni in #6049 🎉