explore extending branchwater plugin to load zip files for prefetch and gather, entirely on rust side?

[ctb]

this could be a good & substantive next step towards higher-level oxidation of sourmash internals - what if we added a load_from plugin entry point to branchwater that loaded zip files, and supported the Index API / passed the index API tests?

this could potentially then support multithreaded prefetch, and hence faster gather.

it would take a while to work, but we could do it incrementally, I suspect...

related issues:

• https://github.com/sourmash-bio/sourmash/issues/1939