discovery: support optional callback for information

Right now, the discovery process offered by `filter_known_objects`
returns all results after the discovery is complete. The new callback
provides a way to get information "in real time" which is useful for at
least a couple of planned use case in the SWH scanner:
- displaying progress information while processing
- update a graphical UI in real time.

This simple callback fits this need without too much troubles.

For some reason, mypy complained about the existing type hint in this
file for unclear reason. So I fixed them.