Explore: Add Mixed Datasource (#53429)

* 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>

* Add more logic around mixed datasource being off for explore

* Build out logic to handle different datasource scenarios

* Add tests

* Finalize last test

* Fix mixed URL with mixed ds off, and relevant test

* Fix datasource to explore workflow

* Add datasource change function, call import queries if needed

* add logic for changing single query ds

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

docs/sources/explore/_index.md

@@ -73,3 +73,7 @@ The Share shortened link capability allows you to create smaller and simpler URL
 > **Note:** Available in Grafana 8.5.0 and later versions.
 
 Enabled by default, allows users to create panels in dashboards from within Explore.
+
+### exploreMixedDatasource
+
+Disabled by default, allows users in Explore to have different datasources for different queries. If compatible, results will be combined.