Allow templating Slack NotifyBroadcast

[mkantzer]

I've got a slack notification that I send that alerts me to argocd events, and I'm using grouping keys to thread related notifications.

However, I only want to use notifyBroadcast under certain circumstances.

Currently, you cannot use any templating on that field: I can't do, for example:

    template.app-health-degraded-every: |
      slack:
        notifyBroadcast: {{ if .isProduction }}true{{ else }}false{{ end }}
        deliveryPolicy: PostAndUpdate

I have to provide a concrete true or false.

I recognize that there's likely some string vs bool typing issues here, but it would be really great if we could add some text templating to include notifyBroadcast. (And maybe deliveryPolicy?)

I'm also happy to have this be accomplished in any other way (perhaps a new field notifyBroadcastExpr where you can provide an expr test, similar to how the triggers are executed?), as long as I can re-use a template but change these settings.