Support C++20 std::format as an alternative to fmtlib

include/spdlog/sinks/ringbuffer_sink.h

@@ -50,7 +50,11 @@ public:
         {
             memory_buf_t formatted;
             base_sink<Mutex>::formatter_->format(q_.at(i), formatted);
+#ifdef SPDLOG_USE_STD_FORMAT
+            ret.push_back(std::move(formatted));
+#else
             ret.push_back(fmt::to_string(formatted));
+#endif
         }
         return ret;
     }