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/manage-dashboards/DashboardImportCtrl.test.ts

@@ -31,7 +31,7 @@ describe('DashboardImportCtrl', () => {
       config.datasources = {
         ds: {
           type: 'test-db',
-        },
+        } as any,
       };
 
       ctx.ctrl.onUpload({