use xarray.open_zarr and make aiohttp and s3fs optional

[vincentsarago]

This PR does:

• remove reference and consolidated options
• fall back to open_zarr when not dealing with netcdf files
• make aiohttp and s3fs optional dependencies

Note:

• we could also make h5netcdf and zarr optional but for this first version I think it's better to at least have something that support both format
• in Theory, a user could provider its own opener method (Callable[..., xarray.Dataset]) making all the custom xarray dataset opener optional

[abarciauskas-bgse]

This could still work with kerchunk references, do you know? If not I will try to test it. It may not matter if we go completely with icechunk stores (which should have the same access API as vanilla zarr stores) but it would be good to know if we were losing support for kerchunk references.