Plugins: Always validate root URL if specified in signature manfiest (#52332)

* always validate root URL if specified in signature

* tidy imports

pkg/plugins/manager/testdata/invalid-v2-extra-file/plugin/plugin.json

@@ -0,0 +1,16 @@
+{
+  "type": "datasource",
+  "name": "Test",
+  "id": "test",
+  "backend": true,
+  "executable": "test",
+  "state": "alpha",
+  "info": {
+    "version": "1.0.0",
+    "description": "Test",
+    "author": {
+      "name": "Will Browne",
+      "url": "https://willbrowne.com"
+    }
+  }
+}