Callbacks service docs updated

api/edge/callbacks/internal/model/callback.go

@@ -0,0 +1,8 @@
+package model
+
+import pmodel "github.com/tech/sendico/pkg/model"
+
+type CallbackInternal struct {
+	pmodel.Callback `bson:",inline" json:",inline"`
+	SecretRef       string `bson:"secretRef"`
+}