AlertingNG: POC of evaluator under feature flag. (#27922)

* New feature toggle for enabling alerting NG

* Initial commit

* Modify evaluate alert API request

* Check for unique labels in alert execution result dataframes

* Remove print statement

* Additional minor fixes/comments

* Fix lint issues

* Add API endpoint for evaluating panel queries

* Push missing renaming

* add refId for condition to API

* add refId for condition to API

* switch dashboard based eval to get method

* add from/to params to dashboard based eval

* add from/to params to  eval endpoint

Co-authored-by: kyle <kyle@grafana.com>

docs/sources/administration/configuration.md

@@ -1417,7 +1417,7 @@ For more information about Grafana Enterprise, refer to [Grafana Enterprise]({{<
 
 ### enable
 
-Keys of alpha features to enable, separated by space. Available alpha features are: `transformations`
+Keys of alpha features to enable, separated by space. Available alpha features are: `transformations`,`ngalert`
 
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