Write zarr encoding type

[Zethson]

AnnData writes the encoding_type into the zarr file https://github.com/scverse/anndata/blob/420191fb11afc36a58a5950798af7f695e18def4/src/anndata/_io/specs/registry.py#L72 which makes it very easy to identify whether a written zarr file is an AnnData object or not.

Would it be possible or a good idea to do the same for SpatialData zarr files?

[LucaMarconato]

Closing as for the moment we rely on the .zattrs metadata, which contains the needed information. More explained here: #891 (comment)