Alerting: Add endpoint for querying state history (#62166)

* Define endpoint and generate

* Wire up and register endpoint

* Cleanup, define authorization

* Forgot the leading slash

* Wire up query and SignedInUser

* Wire up timerange query params

* Add todo for label queries

* Drop comment

* Update path to rules subtree

public/api-merged.json

@@ -19076,6 +19076,7 @@
       }
     },
     "gettableAlerts": {
+      "description": "GettableAlerts gettable alerts",
       "type": "array",
       "items": {
         "$ref": "#/definitions/gettableAlert"