Backend Plugins: Refactor backend plugin registration and start (#21452)

Moves the details of loading plugins into the backend
plugin manager from the respective plugin (datasource,
transform and renderer).

pkg/plugins/app_plugin.go

@@ -46,7 +46,7 @@ type JwtTokenAuth struct {
 }
 
 func (app *AppPlugin) Load(decoder *json.Decoder, pluginDir string) error {
-	if err := decoder.Decode(&app); err != nil {
+	if err := decoder.Decode(app); err != nil {
 		return err
 	}