Chore: Refactor backend plugin manager/tsdb query data (#34944)

Move QueryData method into backend plugin manager which HandleRequest uses to 
query data from plugin SDK supported data sources. This allowed us to remove a lot 
of code no longer needed.

Ref #21510

Co-authored-by: Will Browne <wbrowne@users.noreply.github.com>

pkg/plugins/app_plugin.go

@@ -71,7 +71,7 @@ func (app *AppPlugin) Load(decoder *json.Decoder, base *PluginBase, backendPlugi
 	if app.Backend {
 		cmd := ComposePluginStartCommand(app.Executable)
 		fullpath := filepath.Join(base.PluginDir, cmd)
-		factory := grpcplugin.NewBackendPlugin(app.Id, fullpath, grpcplugin.PluginStartFuncs{})
+		factory := grpcplugin.NewBackendPlugin(app.Id, fullpath)
 		if err := backendPluginManager.RegisterAndStart(context.Background(), app.Id, factory); err != nil {
 			return nil, errutil.Wrapf(err, "failed to register backend plugin")
 		}