feat: multiple outputs

[ilan-gold]

This is a naive implementation of outputting multiple data matrices and represents a breaking change.

As an example, the outputs for an AnnData object with layers["sparse"] and obsm["pca"] would be a dict:

{
    "layers.sparse": ...,
    "obsm.pca": ...,
    "obs": ...,
    ...
}

This is the same string syntax from scanpy (scverse/scanpy#4359) and also nicely leaves the old behavior in tact since X will still map to the string X in the dict (and if using the raw zarr dataset API).

Resolves #252's ask.