Move file iteration code to the FootprintGenerator class

@johnbeard This moves more hyperduplicated code into the FootprintGenerator class

Howto use

1. Set files nargs='*' as by default this will generate everything (recursive search for .yaml in .)
2. For example for the NoLead generator: Checkout from !1120 (merged) & replace the args.files loop by

    FootprintGenerator.run_on_files(
        NoLeadGenerator,
        args,
        configuration=configuration,
    )

kwargs will be passed so this is compatible

This MR will cause merge conflicts with !1120 (merged) . Therefore, I propose to first merge this part and migrate the generators later.