scripts: add dump_bugs_pickle.py

This prints the contents of a zephyr-bugs-$DATE.pickle file in a
format similar to the output of list_issues.py.

It will be useful to have this in tree so that we can better record
the known issues at the time of a particular zephyr release.

The pickle file itself should be created using the bug snapshot
workflow defined in .github/workflows/bug_snapshot.yaml, which can be
triggered manually from this URL:

https://github.com/zephyrproject-rtos/zephyr/actions/workflows/bug_snapshot.yaml



Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>