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

apps/plugins/pkg/apis/plugins/v0alpha1/pluginmeta_spec_gen.go

@@ -40,6 +40,7 @@ type PluginMetaJSONData struct {
 	SkipDataQuery *bool                    `json:"skipDataQuery,omitempty"`
 	State         *PluginMetaJSONDataState `json:"state,omitempty"`
 	Streaming     *bool                    `json:"streaming,omitempty"`
+	Suggestions   *bool                    `json:"suggestions,omitempty"`
 	Tracing       *bool                    `json:"tracing,omitempty"`
 	Iam           *PluginMetaIAM           `json:"iam,omitempty"`
 	// +listType=atomic