Enumerate categories

The Geocoding API has introduced experimental support for category search, which is analogous to MapKit’s MKLocalSearch/MKLocalSearchRequest API. The Geocoding API doesn’t currently support natural language queries; instead, you specify one of a number of predefined keywords as the query and pointOfInterest as the scope (and typically a focalLocation). An application can present the user with a series of buttons for common categories to search for, similar to the one in Apple’s Maps application.

Category search currently works without any modifications to this library. However, we can improve the feature’s discoverability by providing an enumeration of the supported categories. Since the set of categories is open-ended and subject to change, it’ll need to be an extensible string enumeration, similar to the profile identifiers exposed by MapboxDirections.swift. We should mark this enumeration with the - experimental documentation command until the “Preview” tag is removed from the API documentation.

/cc @frederoni @bsudekum

Start by reviewing the profile identifier enumeration pattern in MapboxDirections.swift and MBRouteOptions.h, then compare it with the Geocoding API’s category-search documentation. Done means exposing an extensible category string enumeration and marking it with the experimental documentation command until the API is no longer in Preview.