Allow per display data sources in Grafana agent

[arlake228]

The pSConfig Grafana agent currently configures a display's data source by using one of the auto-created ones or you can specify "manual" and it will use the globally configured "grafana-datasource-name" It shouldn't be too hard to add an option under display with grafana-datasource-name. The use case is that in the WLCG's schema, they use different data sources for each data type.