Add a Labels group object for v05

[dstansby]

This is an alternative to #110 in how to structure v05. In particular, instead of copy/pasting all of v04, this instead imports and modifies classes from v04 as needed. For this minimal example, this just imports and re-uses ome_zarr_models.v04.labels.LabelsAttrs, but I think for the more complex classes this will lead to saving quite a few duplicated lines of code.

I've also added a ome_attributes property to BaseGroupv05. My idea here was to add the same property to BaseGroupv04, giving a consistent way to get at the OME specific attributes, regardless of whether they're under "ome" (v05) or not (v04).

I think I like this more than #110, so will let this sit for a little while and merge it unless there's any feedback/suggestions otherwise.