Add the ability to cancel a rollout

[sledigabel]

We need a way to signal to the APS that a progressive rollout is cancelled by a user.

That includes a new final state (Cancelled) and some way of notifying the APS object of the cancellation.

[maruina]

What's the difference with https://github.com/Skyscanner/applicationset-progressive-sync/issues/14 ?

[sledigabel]

mmmh good point. I think the difference is that pause implies a resume somehow, whereas cancel is actually cancelled for good. it could be restarted but not resumed.

It could be just semantics here, the idea was to provide some way for a user to cancel an ongoing deployment. If we want to make it part of pause, or expand pause with an extra use case, I'm all up for no duplicate effort