maybe add new command, sourmash prefetch

[ctb]

I'm a big fan of the prefetch_gather functionality in genome-grist that pulls out overlapping signatures from a large database for the purpose of doing downstream gather and other things, and it seems like #1238 is going to be adding the related functionality from greyhound #1226 into sourmash. However, I am wondering if there's value in explicitly adding a Python-based sourmash CLI function, sourmash prefetch, that would let us explore the behavior in non-Rust code. It seems like it would be generically useful to have it be available, and it would also prevent us from copy-pasting prefetch_gather over to other projects like charcoal and maybe thumper...

[luizirber]

As supporting datapoint, yesterday I worked a bit on bringing the counter gather from greyhound into Python, and there is a bit of the functionality that is shared with prefetch.

I think it's small enough to be available in sourmash, especially since it's being copy-and-pasted into multiple projects 🙃

[ctb]

idle early morning thought - double check that the prefetch approach works for similarity and max_containment too.

[ctb]

also struck by the basic similarity between the behavior of prefetch and search --containment.