[WIP] Plugins: Refactoring backend initialization flow (#42247)

* refactoring store interface and init flow

* fix import

* fix linter

* refactor resource calling

* load with class

* re-order args

* fix tests

* fix linter

* remove old creator

* add custom config struct

* fix some tests

* cleanup

* fix linter

* add connect failure error

* remove unused err

* convert test over

pkg/plugins/manager/testdata/test-app-with-includes/dashboards/memory.json

@@ -0,0 +1,5 @@
+{
+  "title": "Nginx Memory",
+  "revision": 2,
+  "schemaVersion": 11
+}