consider importing rust code improvements from `sra_search`?

[ctb]

in https://github.com/sourmash-bio/sra_search/pull/12, @luizirber defines some functions that seem to me like they would be nice to have in sourmash core -

• fn check_compatible_downsample - check if two KerMinHash are compatible w/downsampling
• fn prepare_query - downsample a sketch as needed (a replacement for sig.select_sketch

maybe we should consider bringing them in here?

[luizirber]

Note that they were both borrowed from #2230, so technically they will go in at some point. But can also make a smaller PR