Dashboards: Data source template variable options now specify a current value using uid. (#69259)

* feat: template data source option current by uid

* fix: e2e with explicit uid for "slow-prometheus"

* revert: unrelated change

* revert: unreverted the actually related change

* chore: e2e support for testing ${variable:text}

* fix: use `:raw` instead of `:value` in e2e dashboard

devenv/datasources.yaml

@@ -63,6 +63,7 @@ datasources:
       prometheusVersion: 2.40.0
 
   - name: gdev-slow-prometheus
+    uid: gdev-slow-prometheus-uid
     type: prometheus
     access: proxy
     url: http://localhost:3011