Add missing s3 mock

Author: pixcc

ydb/core/wrappers/ut_helpers/s3_mock.h

@@ -71,6 +71,7 @@ class TS3Mock: public THttpServer::ICallBack {
     const char* GetError();
 
     const THashMap<TString, TString>& GetData() const { return Data; }
+    THashMap<TString, TString>& GetData() { return Data; }
 
 private:
     const TSettings Settings;