Provide an option for the user to select the severity of alerts to be sent to Grafana OnCall

- Currently, the severity of alerts sent to Grafana is determined [here](https://github.com/stakater/grafana-cloud-ansible-operator/blob/main/roles/grafana_cloud_operator/tasks/modify_alertmanager_secret.yml#L19).
- Provide a field in the GCO CR for the user to provide options of severity
  - eg: only send warning and critical but not info
