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.go

@@ -17,7 +17,6 @@ import (
 	"github.com/prometheus/common/model"
 	"gopkg.in/yaml.v3"
 
-	"github.com/grafana/grafana/pkg/components/simplejson"
 	"github.com/grafana/grafana/pkg/services/ngalert/models"
 	"github.com/grafana/grafana/pkg/services/secrets"
 	"github.com/grafana/grafana/pkg/util"
@@ -964,12 +963,54 @@ func AllReceivers(route *config.Route) (res []string) {
 	return res
 }
 
+type RawMessage json.RawMessage // This type alias adds YAML marshaling to the json.RawMessage.
+
+// MarshalJSON returns m as the JSON encoding of m.
+func (r RawMessage) MarshalJSON() ([]byte, error) {
+	return json.Marshal(json.RawMessage(r))
+}
+
+func (r *RawMessage) UnmarshalJSON(data []byte) error {
+	var raw json.RawMessage
+	err := json.Unmarshal(data, &raw)
+	if err != nil {
+		return err
+	}
+	*r = RawMessage(raw)
+	return nil
+}
+
+func (r *RawMessage) UnmarshalYAML(unmarshal func(interface{}) error) error {
+	var data interface{}
+	if err := unmarshal(&data); err != nil {
+		return err
+	}
+	bytes, err := json.Marshal(data)
+	if err != nil {
+		return err
+	}
+	*r = bytes
+	return nil
+}
+
+func (r RawMessage) MarshalYAML() (interface{}, error) {
+	if r == nil {
+		return nil, nil
+	}
+	var d interface{}
+	err := json.Unmarshal(r, &d)
+	if err != nil {
+		return nil, err
+	}
+	return d, nil
+}
+
 type GettableGrafanaReceiver struct {
 	UID                   string            `json:"uid"`
 	Name                  string            `json:"name"`
 	Type                  string            `json:"type"`
 	DisableResolveMessage bool              `json:"disableResolveMessage"`
-	Settings              *simplejson.Json  `json:"settings"`
+	Settings              RawMessage        `json:"settings,omitempty"`
 	SecureFields          map[string]bool   `json:"secureFields"`
 	Provenance            models.Provenance `json:"provenance,omitempty"`
 }
@@ -979,7 +1020,7 @@ type PostableGrafanaReceiver struct {
 	Name                  string            `json:"name"`
 	Type                  string            `json:"type"`
 	DisableResolveMessage bool              `json:"disableResolveMessage"`
-	Settings              *simplejson.Json  `json:"settings"`
+	Settings              RawMessage        `json:"settings,omitempty"`
 	SecureSettings        map[string]string `json:"secureSettings"`
 }