RFE: add a file verify hook to the plugin API

[pmatilai]

As plugins can add new kinds of metadata to files (selinux, ima, fsverity...) there also needs to be a way for them to verify that data. Rpm itself can't because it doesn't know about the metadata in the first place.

[pmatilai]

The biggest problem with this one is returning the verify result: rpmfilesVerify() can only return errors through the predefined rpmVerifyAttrs_e bitfield, whereas arbitrary plugins will need to be able to report arbitrary errors. So we'll need a better verify API to start with.