Enable artifact.open() for tiledbsoma backends

[falexwolf]

In the Python API, we allow to artifact.open() tiledbsoma stores.

It'd be great if we could replace the call of open_soma

census <- open_soma()

with this:

census <- artifact.open()

This is how it looks in Python: https://docs.lamin.ai/cellxgene#slice-a-concatenated-array

To make that happen, it'll only be a few lines of changes to the census package:

https://github.com/chanzuckerberg/cellxgene-census/blob/f32c2748e9664cc196bb1758acb22f5a460fe188/api/r/cellxgene.census/R/open.R#L24C1-L38C2

In particular, replacing the s3 path (uri in the code there) via artifact$path so that it works for any tiledbsoma store.