[Photon] Improve locality for city layer & Add layer & radius query params

[ybert]

The idea here is to get the city in the PhotonAddress Model when the object returned by Photon is of type "city".
For example : https://photon.komoot.io/reverse?lon=13.38890&lat=52.51644&lang=default&limit=1&layer=city&radius=10

In this example, we don't have the field city returned by Photon because it is the city element itself (so in this case the city/locality name is in the name field).

I also added :

• support for radius and layer query params as documented here https://github.com/komoot/photon
• wrapping type in PhotonAddress model

[jbelien]

Thanks for you contribution! Looks good 👍

Could you rebase your PR to our master branch so I can approve and merge it? Thanks.