support taxonomy format-loading plugins

[ctb]

(discussion points moved here from #1353)

it would be nice to be able to load taxonomies in different formats (e.g. greengenes) 'natively' within sourmash. I think a good way to do this would be to generalize the loading code under tax/ in the same way that we generalized the loading code for sketches from files/etc in https://github.com/sourmash-bio/sourmash/pull/2428/files (see save_load.py).

There is code in #2538 for parsing greengenes.

also we could load taxonomies via URL (with fsspec).