Add a webhook for finalizing

[cdlliuy]

Describe the feature

Flagger provides different kinds of webhooks to control the rollout process.

Besides all current webhooks, per our business requirement, we hope to have a webhook between Promotion and Finalizing steps.

In promotion step, flagger copies the canary deployment spec to primary.
In Finalizing step, flagger scales down the canary deployment.
Anyway, in between, we would like to a webhook to provide an approval before the "scale-down-to-zero" operation.

I will raise a PR to achieve this feature for review.