[grafana] add support for envValueFrom in sidecar.alerts both initContainer and watch container

[peter-kyu]

We are using xxx-sc-alerts to watch for modifications to the ConfigMap in real time. However, the Grafana Helm chart currently only supports the envValueFrom option for the xxx-init-sc-alerts container. Therefore, I made some modifications to the xxx-sc-alerts container as well.

Additionally, I added the tpl function to the searchNamespace option, following the pattern used in other containers.

Please take a look. thanks

[CLAassistant]

All committers have signed the CLA.