Implement confirm-promotion hook

[stefanprodan]

This PR implements the confirm-promotion webhook type for canaries, A/B testing and Blue/Green deployments. The confirm promotion hooks are executed right before the promotion step. The canary promotion is paused until the hooks return HTTP 200. While the promotion is paused, Flagger will continue to run the metrics checks and load tests. If you have notifications enabled, Flagger will post a message to Slack or MS Teams when a canary promotion is waiting for approval.

Example:

    webhooks:
      - name: "start analysis gate"
        type: confirm-rollout
        url: http://flagger-loadtester.test/gate/approve
      - name: "load testing"
        type: rollout
        url: http://flagger-loadtester.test/
        timeout: 5s
        metadata:
          cmd: "hey -z 1m -q 10 -c 2 http://podinfo-canary.test:9898/"
      - name: "promote gate"
        type: confirm-promotion
        url: http://flagger-loadtester.test/gate/approve

Fix: #299

Flagger image: weaveworks/flagger:confirm-promotion-8282f86