PDP Epic

https://github.com/camunda/product-hub/issues/990

Also do OPT-6484 as part of this.

Testing Notes

• Requirements:
  • to receive an onboarding notification there needs to be a new process for which the first process instance has been started
  • Notifications are only available in SaaS
  • The controller changes to set the required env vars are merged already so this should be testable with a dev cloud generation

Use cases to test:

• Happy path: receive a notification when a new instance for a new process was started, notification links to instant preview dashboard
• User who does not have permission to access Optimize does not receive any notifications
• Only one notification per process: New instances after the very first one that triggered the notification should not trigger additional notifications
• Issues during the notification sending are logged and don't cause any other problems (eg notificaiton API not available, env vars incorrectly configured)