Add pre and post rollout hooks

[stefanprodan]

This PR introduces two types of web hooks:

• pre-rollout hooks are executed before routing traffic to canary. The canary advancement is paused if a pre-rollout hook fails and if the number of failures reach the threshold the canary will be rollback.
• post-rollout hooks are executed after the canary has been promoted or rolled back. If a post rollout hook fails the error is logged.

Example:

apiVersion: flagger.app/v1alpha3
kind: Canary
spec:
  canaryAnalysis:
    webhooks:
      - name: "smoke test"
        type: pre-rollout
        url: http://some.service:8080/
        timeout: 5s
        metadata:
          some: "data"
      - name: "load test"
        type: rollout
        url: http://flagger-loadtester.test/
        timeout: 5s
        metadata:
          cmd: "hey -z 1m -q 5 -c 2 http://podinfo-canary.test:9898/"
      - name: "notify"
        type: post-rollout
        url: http://telegram.bot:8888/
        timeout: 5s
        metadata:
          some: "message"

If the webhook type is not specified it will default to rollout making this change backwards compatible.

Fix: #117