Alerting: replace usage of simplejson to json.RawMessage in NotificationChannelConfig (#60423)

* introduce alias for json.RawMessage with name RawMessage. This is needed to keep raw JSON and implement a marshaler for YAML, which does not seem to be used but there are tests that fail.
* replace usage of simplejson with RawMessage in NotificationChannelConfig
* remove usage of simplejson in tests
* change migration code to convert simplejson to raw message

pkg/services/ngalert/api/tooling/definitions/alertmanager_test.go

@@ -1040,3 +1040,49 @@ func Test_Marshaling_Validation(t *testing.T) {
 	expected := []model.LabelName{"alertname"}
 	require.Equal(t, expected, tmp.AlertmanagerConfig.Config.Route.GroupBy)
 }
+
+func Test_RawMessageMarshaling(t *testing.T) {
+	type Data struct {
+		Field RawMessage `json:"field" yaml:"field"`
+	}
+
+	t.Run("should unmarshal nil", func(t *testing.T) {
+		v := Data{
+			Field: nil,
+		}
+		data, err := json.Marshal(v)
+		require.NoError(t, err)
+		assert.JSONEq(t, `{ "field": null }`, string(data))
+
+		var n Data
+		require.NoError(t, json.Unmarshal(data, &n))
+		assert.Equal(t, RawMessage("null"), n.Field)
+
+		data, err = yaml.Marshal(&v)
+		require.NoError(t, err)
+		assert.Equal(t, "field: null\n", string(data))
+
+		require.NoError(t, yaml.Unmarshal(data, &n))
+		assert.Nil(t, n.Field)
+	})
+
+	t.Run("should unmarshal value", func(t *testing.T) {
+		v := Data{
+			Field: RawMessage(`{ "data": "test"}`),
+		}
+		data, err := json.Marshal(v)
+		require.NoError(t, err)
+		assert.JSONEq(t, `{"field":{"data":"test"}}`, string(data))
+
+		var n Data
+		require.NoError(t, json.Unmarshal(data, &n))
+		assert.Equal(t, RawMessage(`{"data":"test"}`), n.Field)
+
+		data, err = yaml.Marshal(&v)
+		require.NoError(t, err)
+		assert.Equal(t, "field:\n    data: test\n", string(data))
+
+		require.NoError(t, yaml.Unmarshal(data, &n))
+		assert.Equal(t, RawMessage(`{"data":"test"}`), n.Field)
+	})
+}