Add Helm chart for iGotify deployment

[cellexec]

✨ Summary

This pull request adds Helm support for deploying the iGotify Notification Manager alongside the original Gotify server as a dependency.

🛠️ Key Changes

• Introduced a Helm chart for easier and customizable deployment.

• Added the appuser with UID 1000 to the container image to support Kubernetes' security context configuration:

  securityContext:
    runAsNonRoot: true
    runAsUser: 1000
  

🐣 This is my first pull request on GitHub. Feedback is always very welcome!