Alerting: Change __value__ label to __value_string__ annotation and add ValueString variable in notifications (#36032)

* Alerting: Allow __value__ label in notifications
was being removed by removePrivateItems
discoverd in #36020, but issue is not about that specifically

* __value__ label to __value_string__ annotation
and .ValueString extended property for notifications

pkg/tests/api/alerting/api_notification_channel_test.go

@@ -1455,9 +1455,10 @@ var expNotifications = map[string][]string{
 			  },
 			  "annotations": {},
 			  "startsAt": "%s",
+        "valueString": "[ var='A' labels={} value=1 ]",
 			  "endsAt": "0001-01-01T00:00:00Z",
 			  "generatorURL": "http://localhost:3000/alerting/UID_WebhookAlert/edit",
-			  "fingerprint": "7611eef9e67f6e50",
+			  "fingerprint": "929467973978d053",
 			  "silenceURL": "http://localhost:3000/alerting/silence/new?alertmanager=grafana&matchers=alertname%%3DWebhookAlert",
 			  "dashboardURL": "",
 			  "panelURL": ""
@@ -1621,10 +1622,11 @@ var expNotifications = map[string][]string{
 		  {
 			"labels": {
 			  "__alert_rule_uid__": "UID_AlertmanagerAlert",
-              "__value__": "[ var='A' labels={} value=1 ]",
 			  "alertname": "AlertmanagerAlert"
 			},
-			"annotations": {},
+			"annotations": {
+        "__value_string__": "[ var='A' labels={} value=1 ]"
+      },
 			"startsAt": "%s",
 			"endsAt": "0001-01-01T00:00:00Z",
 			"generatorURL": "http://localhost:3000/alerting/UID_AlertmanagerAlert/edit",