Add support for managing external gateways

This change implements the logic to call the new API for managing
external gateways.

Relevant Neutron core change:
https://review.opendev.org/c/openstack/neutron/+/873593

Co-Authored-by: Frode Nordahl <frode.nordahl@canonical.com>
Related-Bug: #2002687
Change-Id: Ib45f30f552934a0a5c035c3b7fadfc0d522219ba

Author: Dmitrii Shcherbakov