Notification Engine should support a list of triggers from a single annotation

[RaviHari]

ref: argoproj/argo-rollouts#1783

Notification engine should handle multiple triggers in a given annotation:

apiVersion: argoproj.io/v1alpha1
kind: Rollout
metadata:
  annotations:
    notifications.argoproj.io/subscribe.on-rollout-completed,on-rollout-updated,on-rollout-pausedon-rollout-aborted,on-rollout-step-completed,on-scaling-replica-set.slack: ar-notifications-test

[jessesuen]

@alexmt can you make these equivalent?

apiVersion: argoproj.io/v1alpha1
kind: Rollout
metadata:
  annotations:
    notifications.argoproj.io/subscribe.on-XXXXX.slack: ar-notifications-test
    notifications.argoproj.io/trigger-group.XXXXX: on-rollout-completed,on-rollout-updated,on-rollout-pausedon-rollout-aborted,on-rollout-step-completed,on-scaling-replica-set

  notifications.argoproj.io/subscribe_yaml: |
    - trigger: my-trigger1, my-trigger2, my-trigger3
      destinations:
        - service: my-service1
          recipients:
            - recipient-1
            - recipient-2

[alexmt]

Here it is:

 notifications.argoproj.io/subscribe_yaml: |
    - trigger: [on-rollout-completed,on-rollout-updated,on-rollout-pausedon-rollout-aborted,on-rollout-step-completed,on-scaling-replica-set]
      destinations:
        - service: slack
          recipients: [ar-notifications-test]