Versioned notifications for audits

Following https://blueprints.launchpad.net/watcher/+spec/watcher-versioned-objects and https://blueprints.launchpad.net/watcher/+spec/watcher-notifications-ovo , Watcher now has the required building blocks to now focus on what notifications we want to send during the life-cycle of an audit.

This blueprint should define the list of notifications to be implemented as well as their respective payload structures.

This featured should take inspiration from the work that has been done on the matter by the Nova team in https://specs.openstack.org/openstack/nova-specs/specs/mitaka/implemented/versioned-notification-api.html and https://specs.openstack.org/openstack/nova-specs/specs/newton/approved/versioned-notification-transformation.html .