Alerting: Allow customization of Google chat message (#43568)

* Allow customizable googlechat message via optional setting

* Add optional message field in googlechat contact point configurator

* Fix strange error message on send if template fails to fully evaluate

* Elevate template evaluation failure logs to Warn level

* Extract default.title template embed from all channels to shared constant

pkg/tests/api/alerting/api_available_channel_test.go

@@ -1487,6 +1487,22 @@ var expAvailableChannelJsonOutput = `
         "required": true,
         "validationRule": "",
         "secure": false
+      },
+      {
+        "element": "textarea",
+        "inputType": "",
+        "label": "Message",
+        "description": "",
+        "placeholder": "{{ template \"default.message\" . }}",
+        "propertyName": "message",
+        "selectOptions": null,
+        "showWhen": {
+          "field": "",
+          "is": ""
+        },
+        "required": false,
+        "validationRule": "",
+        "secure": false
       }
     ]
   },