add create user method

api/smm_core.proto

@@ -16,6 +16,14 @@ service SmmCore {
         };
     }
 
+    // users
+    rpc AddUser(CreateUserReq) returns (User) {
+      option (google.api.http) = {
+        post: "/users",
+        body: "*"
+      };
+    }
+
     // categories
     rpc AddCategory(CreateCategoryReq) returns (Category) {
       option (google.api.http) = {
@@ -39,6 +47,15 @@ message PingReq {}
 
 message PingRsp {}
 
+message CreateUserReq {
+  string username = 1;
+}
+
+message User {
+  int32 id = 1;
+  string username = 2;
+}
+
 message CreateCategoryReq {
   string name = 1;
   int32 user_id = 2;