Support environment variable substitution in Grafana dashboards

<!-- Please only use this template for submitting feature requests --> **Why is this needed**: When Grafana is deployed using the official Helm chart ([grafana/helm-charts](https://github.com/grafana/helm-charts/tree/main/charts/grafana)), dashboards are typically managed and version-controlled as part of Infrastructure-as-Code (IaC). In multi-cluster or multi-environment setups, dashboards often contain cluster-specific or environment-specific values such as AWS Account ID, region, or log group name. Currently, there is no supported way to pass Helm values or environment variables into Grafana dashboard variables. This forces teams to duplicate the same dashboard per cluster with hardcoded values, breaking the principle of reusable IaC components. A mechanism to inject environment-specific variables from Helm values (or environment variables) into dashboard variables would allow a single dashboard definition to be shared safely across clusters and environments. **What would you like to be added**: Add native support for environment variable substitution or Helm value injection in Grafana dashboards. **Who is this feature for?** This feature is intended for DevOps, Platform, and SRE teams who deploy Grafana via Helm and manage all configuration as code (IaC). It enables reusable, parameterized dashboards across multiple clusters or environments without duplicating dashboards for each configuration. Ref: https://github.com/grafana/grafana/issues/25443