Add optional datasource UID

[ben-nelson-nbcuni]

Using a datasource UID is best practice for provisioned dashboards that will get deployed across multiple Grafana instances / versions. Please add the option to add UID to https://github.com/VictoriaMetrics/helm-charts/blob/master/charts/victoria-metrics-k8s-stack/templates/grafana/datasource.yaml#L21-L27

Normally I'd get around this by disabling your defaults and creating my own ConfigMap, however there isn't a helm option to include the grafana datasources sidecar but not include your default values. You should consider using another flag besides .Values.grafana.sidecar.datasources.enabled to determine whether this ConfigMap gets created.