Refactor: Plugin exports & data source / panel types (#16364)

* wip: began work off removing meta and pluginExports from DataSourceApi interface

* WIP: changing how plugins are exports and loaded

* Down the refactoring rabit hole that keeps expanding

* TestData now returns DataSourcePlugin

* Refactoring: fixed app config page loading, type renamings and more typings

* Refactor: Correct casing on DatasourceStatus => DataSourceStatus

public/app/features/dashboard/dashgrid/PanelPluginNotFound.tsx

@@ -62,9 +62,7 @@ export function getPanelPluginNotFound(id: string): PanelPlugin {
       updated: '',
       version: '',
     },
-
-    exports: {
-      reactPanel: new ReactPanelPlugin(NotFound),
-    },
+    reactPlugin: new ReactPanelPlugin(NotFound),
+    angularPlugin: null,
   };
 }