reverseGeocode should use named parameters and follow API specification

[jt274]

The reverse geocode documentation: https://radar.com/documentation/api#reverse-geocode says there are two input parameters, coordinates and layers.

The flutter documentation here: https://radar.com/documentation/sdk/flutter#geocoding shows a type Map<String, dynamic> input that uses latitude, longitude, and accuracy. There is no description of what accuracy does, one can only assume. There is also no mention of the layers parameter.

The reverseGeocode function should use named parameters, not a Map<String, dynamic> type which can easily be mistyped. It should include:

latitude (type double)
longitude (type double)
layers (type List<Layer>, using enum to avoid mistyped layer names)

Example:

final res = await Radar.reverseGeocode({
  latitude: 11.1234,
  longitude: 22.3456,
  layers: [
    Layer.place,
    Layer.address,
    Layer.state,
  ],
});

Additionally, the return type should not be Map<dynamic, dynamic>? (again to avoid mistyped parameter names), but have its own class.

[KennyHuRadar]

This issues should be addressed here: #56 (comment)