Add support for dumping POI in GeoJSON format

[jpmens]

Add GeoJSON POI only support to the API and ocat.

(Invalidates #435)

[jpmens]

Surfaced via the API and in ocat:

$ ./ocat -u jpm -d 5s --format geojsonpoi --from 2023-12-17 --to 2023-12-21

{
 "type": "FeatureCollection",
 "features": [
  {
   "type": "Feature",
   "geometry": {
    "type": "Point",
    "coordinates": [
     8.572055000000001,
     50.046365
    ]
   },
   "properties": {
    "name": "FRA gate B28, LH498",
    "address": "Terminal 1 Flugsteig B, Ankunftbogen, 60549 Frankfurt, Germany",
    "isotst": "2023-12-17T12:27:43Z"
   }
  },
  {
   "type": "Feature",
   "geometry": {
    "type": "Point",
    "coordinates": [
     -96.717023,
     17.065921
    ]
   },
   "properties": {
    "name": "Oaxaca, hotel Los Pilares",
    "address": "Los Pilares, C las Curtidurías 721-A, Colonia América Norte, 68080 Oaxaca, OAX, Mexico",
    "isotst": "2023-12-20T02:43:42Z"
   }
  },
  {
   "type": "Feature",
   "geometry": {
    "type": "Point",
    "coordinates": [
     -96.724276,
     17.062621
    ]
   },
   "properties": {
    "name": "café Brujula; Pepe Limón",
    "address": "Monte de Piedad, Avda de José María Morelos 703, Oaxaca de Juárez, 68000 Oaxaca de Juárez, OAX, Mexico",
    "isotst": "2023-12-20T19:50:55Z"
   }
  }
 ]
}

[jpmens]

Imported into https://umap.openstreetmap.de/en:

[jpmens]

Completed.