Alerting: Export json encoded fields to HCL using jsonencode function

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: when exporting a grafana alert to HCL, the model is returned as json encoded, this is problematic as we might want to change it in the future without re-exporting it **What would you like to be added**: it would be nicer if the exported rule would be exported with the `jsonencode` function of HCL **Who is this feature for?** people who experiment with alerts in the UI and later exporting them to Terraform code for IaC