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/services/pluginsintegration/loader/fakes.go

@@ -0,0 +1,38 @@
+package loader
+
+import (
+	"context"
+
+	"github.com/grafana/grafana/pkg/plugins"
+)
+
+type fakeSignatureErrorTracker struct {
+	RecordFunc          func(ctx context.Context, err *plugins.SignatureError)
+	ClearFunc           func(ctx context.Context, pluginID string)
+	SignatureErrorsFunc func(ctx context.Context) []*plugins.SignatureError
+}
+
+func newFakeSignatureErrorTracker() *fakeSignatureErrorTracker {
+	return &fakeSignatureErrorTracker{}
+}
+
+func (t *fakeSignatureErrorTracker) Record(ctx context.Context, err *plugins.SignatureError) {
+	if t.RecordFunc != nil {
+		t.RecordFunc(ctx, err)
+		return
+	}
+}
+
+func (t *fakeSignatureErrorTracker) Clear(ctx context.Context, pluginID string) {
+	if t.ClearFunc != nil {
+		t.ClearFunc(ctx, pluginID)
+		return
+	}
+}
+
+func (t *fakeSignatureErrorTracker) SignatureErrors(ctx context.Context) []*plugins.SignatureError {
+	if t.SignatureErrorsFunc != nil {
+		return t.SignatureErrorsFunc(ctx)
+	}
+	return nil
+}