Add iterator-based vulnerability/enrichment writes to the existing DB APIs.

Ensure backward compatibility with non-iterator interfaces.

Quoting hdonnay:

I generally find that channels make for bad API surface because there's no way to communicate callee-cancellation (as in, if the function that has a channel passed in needs to exit early, the goroutine feeding the channel is live until GC or, if written correctly, a context expiration) and forces the use of an additional goroutine in otherwise straight-line code

the rangefunc stuff can still be driven manually, so that would be my preference if you want to pursue the change

Example/reference: https://github.com/quay/claircore/pull/1092/commits/123b1f68fe3fb2f1785de5c03b4ee6c0047976d3