Implement OME-NGFF metadata conversion according to RFC 2

[sbesson]

The current converter code deals exclusively with the transformation of the data from the Zarr v2 from/to the Zarr v3 without modifying the content of the custom group/array attributes.

As per RFC-2, any metadata following the OME-NGFF specification should be transformed as follows:

• the top-level keys (plate, multiscales,...) should be migrated under an ome key
• the version key should be moved under ome

When converting from Zarr v3 to Zarr v2, the attributes should be converted to match the OME-NGFF 0.4 specification.