APK signature verification

I'm not sure how useful would this be, but here goes:

It would be nice to be able to specify an apk signature in addition to package name for verification.

Use case: Allow downloading an APK from any source, ensuring it's still what's expected.

Something like this was added to `fdroidserver` recently, for a similar use-case.

https://gitlab.com/fdroid/fdroidserver/-/merge_requests/984