scripts: set_assignee: add an option to assign module PRs

Add a -m option to set_assignees to have the script find all unassigned
module PRs for areas that have an assignee and set the assignee on those
PRs.

This should help avoiding forgotten PRs on modules. It's implemented as
a search and meant to run periodically, so we don't have to add a
workflow for this on every individual module.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>