Support Define Tenant ID and Client ID for Workload Identity

[jkroepke]

What happened:

At the moment, Grafana fully rely on Environment Variables or grafana.ini for Workload Identity.

What you expected to happen:

I would like to use the same Workload Identity source (token file) in multiple datasource against different Tenant ID. I have multiple Tenants which different Client that trust the OIDC issuer for my AKS cluster. But this requires different Client ID and Tenant ID per datasource settings.

Like the Client ID for Managed Identity, which is also configurable per datasource.