Allow to update secrets

[dtaniwaki]

Is your feature request related to a problem? Please describe.

I updated the slack URL after I tried several notifications with my old incoming webhook URL, but the operator didn't take new one until I delete the pod and let it restarted.

Describe the solution you'd like

We're caching the client once it gets created.

argo-kube-notifier/notification/controller/controller.go

Line 104 in 3b24127

notifierMap[notifier.Name] = integration.NewSlackClient(client, namespace, notifier.Slack)

We need a mechanism to sync secrets and recreate slack clients.

Describe alternatives you've considered

Just delete the operator pod when a secret gets updated although it doesn't scale at all.

Additional context

N/A