`attributes/ome/omero/channels/label` gets incorrectly coerced to integer

[Bisaloo]

Related to Huber-group-EMBL/SpatialData.validate#3. Visible in the blobs example dataset.

  "attributes": {
    "ome": {
      "omero": {
        "channels": [
          {
            "label": 0
          },
          {
            "label": 1
          },
          {
            "label": 2
          }
        ]
      }

Should be

  "attributes": {
    "ome": {
      "omero": {
        "channels": [
          {
            "label": "0"
          },
          {
            "label": "1"
          },
          {
            "label": "2"
          }
        ]
      }

The OME-NGFF spec says attributes/ome/omero/channels/label must be a string, but if it's a string containing just an integer, this is silently converted to an integer, making elements in images/ non-compliant with the OME-NGFF spec.

(I know elements in images/ still won't be completely compliant with the OME-NGFF spec after this because of the transformations that are not available in 0.5, but it's a separate issue.)