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/plugins/models.go

@@ -319,6 +319,7 @@ type PanelDTO struct {
 	HideFromList    bool              `json:"hideFromList"`
 	Sort            int               `json:"sort"`
 	SkipDataQuery   bool              `json:"skipDataQuery"`
+	Suggestions     bool              `json:"suggestions,omitempty"`
 	ReleaseState    string            `json:"state"`
 	BaseURL         string            `json:"baseUrl"`
 	Signature       string            `json:"signature"`