FeatureCollection support

For web mapping purposes, the data returned from Connections will generally have to be transformed into a FeatureCollection (https://tools.ietf.org/html/rfc7946#section-3.3). It's a trivial amount of client-side JS, but as GeoJSON datasets can get very large, it can impact performance/memory use. It would be nice to receive the GeoJSON FeatureCollection directly, with selected fields under `properties`:

```
{
"type": "FeatureCollection",
"features": [
{
"type": "Feature",
"geometry": {
"type": "Point",
"coordinates": [102.0, 0.5]
},
"properties": {
"prop0": "value0"
}
}
]
}
```

FeatureCollections can be fed directly into mapping libraries. Example: https://docs.mapbox.com/mapbox-gl-js/example/geojson-markers/

This is a stretch goal.