Allow canceling in-progress runs

Our notification policy updates a single PR comment per stack per PR. (Also, we trigger proposed runs on every commit to PR that modifies a stack’s root directory.) If a user makes two quick commits in succession (but not pushed at the same time), Spacelift starts 2 runs. The user doesn’t care about the first commit’s proposed run, but we don’t have a way to cancel the run from the first commit from a push policy once the run is in progress.

It’s possible to cancel the in-progress run from the API, so it would be very helpful if we could also cancel in-progress runs via push policies.