Plugins: Add validation stage to plugin loader pipeline (#73053)

* first pass

* change validation signature

* err tracking

* fix

* undo golden

* 1 more

* fix

* adjust doc

* add test helper

* fix linter

pkg/plugins/manager/pipeline/termination/doc.go

@@ -1,5 +1,4 @@
 // Package termination defines the Termination stage of the plugin loader pipeline.
 //
 // The Termination stage must implement the Terminator interface.
-// - Terminate(ctx context.Context, pluginID string) error
 package termination