Make table backend dependencies optional

[imagejan]

Currently, ngio depends on all of anndata, pyarrow, pandas and polars at runtime, leading to a large size of the installed environment.

I suggest making all table backend dependencies optional (and warn when trying to access a table that was written with a backend that's currently unavailable). This would allow using a very "lean" ngio installation just using the "csv" backend, for example.

[lorenzocerrone]

Hi @imagejan, I agree 100%. I started going in the same direction in the last few days. My plan is to make the whole tables module a stand-alone Python package (see https://github.com/BioVisionCenter/ngio-tables).

About the most lean version of it, I think that at minimum either pandas or polars should be there.
My pick for the minimal set of dependencies is:

 - polars
 - zarr
 - pydantic
 - fsspec