Export additional information about OSM objects in Photon API

[lonvia]

Nominatim has the capability to save any OSM tag as extra information with a place object and to return this via the API. Photon currently ignores this information completely even though it might be interesting for special purpose installations (e.g. a search engine for restaurants also wants to return the cuisine information if available.)

Returning arbitrary tags is a bit difficult with ElasticSearch but we could add a fixed set of selected tags. These would not be searchable but only be returned as extra information, just like Nominatim does. I'd suggest an additional property extra, which contains the tags that are available. If there is not extra infromation for a place, the property would be omitted completely, e.g

{
  "features": [
    {
      "geometry": {   "coordinates": [  11.5500582,  48.1778972], "type": "Point"  },
      "type": "Feature",
      "properties": {
        "osm_id": 4068438,
        "country": "Germany",
        "city": "Munich",
        "countrycode": "DE",
        "type": "house",
        "osm_type": "N",
        "osm_key": "amenity",
        "osm_value": "restaurant",
        "name": "Alphut",
        "extra": {
           "cuisine" : "vegan"
        }
      }
    },
      "geometry": {   "coordinates": [  11.5500582,  48.1778972], "type": "Point"  },
      "type": "Feature",
      "properties": {
        "osm_id": 291856,
        "country": "Germany",
        "name": "Munich",
        "osm_type": "place",
        "osm_value": "city"
      }
     }
  ]
}

Implementation steps:

• Provide an additional parameter -extratags for the import that selects the tags to add to the Photon database.
• Dynamically change the mapping to add fields for the additional tags.
• Add extra information to the JSON output.
• Add tests for the new function.

CC @hbruch

[lonvia]

Implemented in #576.