feat(issues): allow opt-in assignment for OOD issues

Allow packagers to decide whether they want to be notified for issues of scope::out-of-date. This change will further enable the ongoing proof of concept work to setup global out of date reporting via a central nvchecker setup.

Fixes: #10 (closed)
Co-authored-by: Robin Candau robincandau@protonmail.com
Signed-off-by: Christian Heusel christian@heusel.eu

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superseeds !23 (closed)