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/api/frontendsettings.go

@@ -164,6 +164,7 @@ func (hs *HTTPServer) getFrontendSettings(c *contextmodel.ReqContext) (*dtos.Fro
 			ModuleHash:      hs.pluginAssets.ModuleHash(c.Req.Context(), panel),
 			BaseURL:         panel.BaseURL,
 			SkipDataQuery:   panel.SkipDataQuery,
+			Suggestions:     panel.Suggestions,
 			HideFromList:    panel.HideFromList,
 			ReleaseState:    string(panel.State),
 			Signature:       string(panel.Signature),