Adds configuration for separate reply-to email header. (#10144)

* Adds configuration for separate reply-to email header.

* Changes config setting name.

* Using a separate variable and value in test.

* Updates for config pointer changes in another PR.

* Adds new key to test config. Adds default value.

api4/system_test.go

@@ -426,6 +426,7 @@ func TestEmailTest(t *testing.T) {
 			SMTPPassword:           model.NewString(""),
 			FeedbackName:           model.NewString(""),
 			FeedbackEmail:          model.NewString(""),
+			ReplyToAddress:         model.NewString(""),
 			SendEmailNotifications: model.NewBool(false),
 		},
 	}