Optionally generate methods for obtaining map keys and list indexes

[andylowry]

In the type configuration used by the generator, a 'map' type is implemented as a map of strings to values, and a collection type is implemented as a collection of values. The overlay adapter method getPathInParent can be used to obtain the map key or list position (though as a string, e.g. "10" not 10). But this is clumsy to code.

We should add a type configuration property like selectorName that, when present, will cause a convenience method to be defined for the object. For example, if we have selectorName: name on a map type, then the generated API for that type will include a method getName() that returns a string. Likewise, selectorName: position on a collection type will give rise to int getPosition().

These methods should return null if the instance does not happen to appear as a MapOverlay or ListOverlay child, and references to the instance, naturally, will reflect its non-reference occurrence (since there will only really be a single instance of the type, and its parent, if any, will be set according to where it was inlined). So e.g. in an OpenAPIv3 document, a Schema object may appear in the /components/schemas map but may also be referenced from various places in the overall model. Its name will be the name appearing in the /components/schemas map.