Plugins: Only load transform plug-ins if expressions feature on (#24110)

* PluginManager: Only load transform plugins if expressions feature on

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>

pkg/setting/setting.go

@@ -289,6 +289,11 @@ type Cfg struct {
 	FeatureToggles map[string]bool
 }
 
+// IsExpressionsEnabled returns whether the expressions feature is enabled.
+func (c Cfg) IsExpressionsEnabled() bool {
+	return c.FeatureToggles["expressions"]
+}
+
 type CommandLineArgs struct {
 	Config   string
 	HomePath string