Alerting: Fix Test Receivers when settings are non-strings (#62156)

* Alerting: Fix Test Receivers when settings are non-strings

As part of the Alerting extraction, we want to make sure we don't have circular depedencies. As such, I had to move `PostableGrafanaReceiver` to a new struct in `grafana/alerting` called `GrafanaReceiver`.

`PostableGrafanaReceiver` has an attribute called `Settings` that uses a Grafana-propietary struct called `RawMessage`, this struct shadows `json.RawMessage`.

When I created `GrafanaReceiver`, I turned settings into a `map[string]string` thinking all settings would end up as strings. This was a mistake, and this test proves that it doesn't work, and breaks the API.

pkg/tests/api/alerting/api_notification_channel_test.go

@@ -98,7 +98,8 @@ func TestIntegrationTestReceivers(t *testing.T) {
 					"type": "email",
 					"disableResolveMessage": false,
 					"settings": {
-						"addresses":"example@email.com"
+						"addresses":"example@email.com",
+						"singleEmail": true
 					},
 					"secureFields": {}
 				}