Suggestions: Deprecate the old API and put external suggestions behind a flag (#114127)

* Suggestions: Deprecate previous API, enable external plugin suggestions behind flag

* fix types for deprecated builder

* restore some support for cloud-onboarding

* add support for cloud-onboarding usage, add test to ensure it keeps working

* refactor to not hardcode on 'core:'

* remove unused import

pkg/services/featuremgmt/toggles_gen.json

@@ -1383,6 +1383,20 @@
         "codeowner": "@grafana/identity-access-team"
       }
     },
+    {
+      "metadata": {
+        "name": "externalVizSuggestions",
+        "resourceVersion": "1763498528748",
+        "creationTimestamp": "2025-11-18T20:42:08Z"
+      },
+      "spec": {
+        "description": "Enable all plugins to supply visualization suggestions (including 3rd party plugins)",
+        "stage": "experimental",
+        "codeowner": "@grafana/dataviz-squad",
+        "frontend": true,
+        "expression": "false"
+      }
+    },
     {
       "metadata": {
         "name": "extraThemes",