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/panel_plugin.go

@@ -8,7 +8,7 @@ type PanelPlugin struct {
 }
 
 func (p *PanelPlugin) Load(decoder *json.Decoder, pluginDir string) error {
-	if err := decoder.Decode(&p); err != nil {
+	if err := decoder.Decode(p); err != nil {
 		return err
 	}