Support OME-NGFF labels metadata

When opening an OME-NGFF image, the [labels metadata](https://ngff.openmicroscopy.org/latest/index.html#labels-md) are currently ignored.

The following example uses [this image](https://ome.github.io/ome-ngff-validator/?source=https://uk1s3.embassy.ebi.ac.uk/idr/zarr/v0.4/idr0079A/idr0079_images.zarr/0/) from the IDR.

[This Neuroglancer link](https://neuroglancer-demo.appspot.com/#!{"layers":[{"name":"image","type":"auto","source":"https://uk1s3.embassy.ebi.ac.uk/idr/zarr/v0.4/idr0079A/idr0079_images.zarr/0"}],"layout":"4panel-alt"}) will open this JSON:
```
{
"layers": [
{
"name": "image",
"type": "auto",
"source": "https://uk1s3.embassy.ebi.ac.uk/idr/zarr/v0.4/idr0079A/idr0079_images.zarr/0"
}
],
"layout": "4panel-alt"
}
```

[Recent improvements](https://github.com/google/neuroglancer/pull/770) from @seankmartin now allow Neuroglancer to open all of the image channels in a very usable way. It would be great to also read any labels and add them as segmentation layers.