Add X-Grafana-User header in case the actor is specified in config

[chaudum]

The X-Grafana-User is and identifier for a single 'user' that is part of a tenant. Grafana Cloud Logs translates the header into the X-Actor-Path that is used in Loki to guarantee query fairness in the scheduler, see https://github.com/grafana/loki/pull/8752