scripts: add github_helpers.py

This is meant to be a place where we can store generic zephyr wrappers
around the de-facto standard github API for python.

The first 'customer' will be a script that snapshots our open bugs at a
particular point in time, which will be added in a later patch.

I think it's useful to factor this file out of there from the
beginning just to keep things clean, even though I don't have a second
customer in mind at the moment.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
Signed-off-by: Stephanos Ioannidis <root@stephanos.io>