provide the method to generate async logger with discard new overflow policy

[wsehjk]

I think it's better to provide create_async_discard_new method to keep consistent with the other two async logger providers create_async and create_async_nb

diff --git a/include/spdlog/async.h b/include/spdlog/async.h
index e96abd19..c0b6bb88 100644
--- a/include/spdlog/async.h
+++ b/include/spdlog/async.h
@@ -57,6 +57,7 @@ struct async_factory_impl {
 
 using async_factory = async_factory_impl<async_overflow_policy::block>;
 using async_factory_nonblock = async_factory_impl<async_overflow_policy::overrun_oldest>;
+using async_factory_discard_new = async_factory_discard_new<async_overflow_policy::discard_new>;
 
 template <typename Sink, typename... SinkArgs>
 inline std::shared_ptr<spdlog::logger> create_async(std::string logger_name,
@@ -71,6 +72,12 @@ inline std::shared_ptr<spdlog::logger> create_async_nb(std::string logger_name,
     return async_factory_nonblock::create<Sink>(std::move(logger_name),
                                                 std::forward<SinkArgs>(sink_args)...);
 }
+template <typename Sink, typename... SinkArgs>
+inline std::shared_ptr<spdlog::logger> create_async_discard_new(std::string logger_name,
+                                                       SinkArgs &&...sink_args) {
+    return async_factory_discard_new::create<Sink>(std::move(logger_name),
+                                                std::forward<SinkArgs>(sink_args)...);
+}