Support compression in sce_to_anndata()

[jashapiro]

Is your feature request related to a problem? Please describe.

We currently write out AnnData files without any compression. We should pass arguments for compression to zellkonverter::writeH5AD()

Describe the solution you'd like

We can add a ... to the call that passes any additional arguments down to zellkonverter::writeH5AD

Describe alternatives you've considered

We could add a add a compression argument that we pass directly to the compression argument, rather than the catchall, but I like the fact that this gives us free support for other writeH5AD() options.

[jashapiro]

We should also allow .h5ad as a file extension for this function, since that seems to be the standard.