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/discovery/doc.go

@@ -1,6 +1,5 @@
 // Package discovery defines the Discovery stage of the plugin loader pipeline.
 
 // The Discovery stage must implement the Discoverer interface.
-// - Discover(ctx context.Context, src plugins.PluginSource) ([]*plugins.FoundBundle, error)
 
 package discovery