feat: API docs for service api (#24425)

Signed-off-by: -LAN- <laipz8200@outlook.com>

api/fields/end_user_fields.py

@@ -1,4 +1,4 @@
-from flask_restx import fields
+from flask_restx import Api, Namespace, fields
 
 simple_end_user_fields = {
     "id": fields.String,
@@ -6,3 +6,7 @@ simple_end_user_fields = {
     "is_anonymous": fields.Boolean,
     "session_id": fields.String,
 }
+
+
+def build_simple_end_user_model(api_or_ns: Api | Namespace):
+    return api_or_ns.model("SimpleEndUser", simple_end_user_fields)