Plugins: Only load transform plug-ins if expressions feature on (#24110)

* PluginManager: Only load transform plugins if expressions feature on

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>

pkg/plugins/testdata/behind-feature-flag/gel/plugin.json

@@ -0,0 +1,13 @@
+{
+  "type": "transform",
+  "name": "GEL",
+  "id": "gel",
+  "backend": true,
+  "info": {
+    "description": "Test",
+    "author": {
+      "name": "Grafana Labs",
+      "url": "https://grafana.com"
+    }
+  }
+}