Explore: Add Mixed Datasource (#51605)

* Toggle on the mixed mode option

* Ensure switching to mixed gives existing query prev datasource

* WIP - Populate datasource when switching between mixed and not

* WIP - handle change from mixed

* Remove preimport filter, refine filter to work for queries

* WIP debugging datasource transition

* Ensure creating a new query gets target data source if switching with no matches between

* Add mixed datasource to rich history display

* Cleanup console logs, add relevant comments

* Add feature toggle for mixed datasource

* Fix Wrapper tests

* Fix tests!

* Fix test types and add feature tracking

* Remove unnecessary default, remove explore/mixed workarounds for D2E

* Move display text logic to mixed datasource file

* Add in the default query parameters to a generated empty query

* Condense some code

* Apply suggestions from code review

Co-authored-by: Giordano Ricci <me@giordanoricci.com>

Co-authored-by: Giordano Ricci <me@giordanoricci.com>

packages/grafana-data/src/types/featureToggles.gen.ts

@@ -44,6 +44,7 @@ export interface FeatureToggles {
   export?: boolean;
   azureMonitorResourcePickerForMetrics?: boolean;
   explore2Dashboard?: boolean;
+  exploreMixedDatasource?: boolean;
   tracing?: boolean;
   commandPalette?: boolean;
   cloudWatchDynamicLabels?: boolean;