Add CLI for conversions from and to standard types (.png, .tiff, .geojson, ...)

[LucaMarconato]

If we have a single image, say a .tiff file, it would be convenient to be able to:

• convert it to a SpatialData Zarr stored object with a single image
• add the image to an existing SpatialData object
  All using the CLI.

[LucaMarconato]

Note, we have a already a parser for .geojson data into a shapes object in ShapesModel.parse(). It could be a good starting point to move that into a CLI function.

Instead, for say .png -> .zarr conversion, the pseudocode would look something like this:

1. use skimage.imread to read the image lazily
2. use Image2DModel.parse to parse the image
3. construct a SpatialData object and write it to disk.

[LucaMarconato]

In particular a very convenient use case would be to quick extract an image in, say, .png, from a Zarr store.