Add config for charmed-openstack-exporter-snap

[gabrielcocenza]

This moves the following files for https://github.com/canonical/charmed-openstack-exporter-snap under control of this automation repo:

• CODEOWNERS
• workflows/promote.yaml

It will also apply our standard config to the charmed-openstack-exporter-snap repository itself.

[chanchiwai-ray]

Error: could not find a branch protection rule with the pattern 'main'

I vaguely remember github terraform cannot set this if you did not set it first cc @jneo8