Add mp3_plugin, support rtsp with mp3 (#4048)

增加mp3_plugin, 解决rtmp推流转rtsp无法播放问题
ref: https://datatracker.ietf.org/doc/html/rfc2250#section-3.5

ext-codec/MP3.h

@@ -0,0 +1,33 @@
+/*
+ * Copyright (c) 2016-present The ZLMediaKit project authors. All Rights Reserved.
+ *
+ * This file is part of ZLMediaKit(https://github.com/ZLMediaKit/ZLMediaKit).
+ *
+ * Use of this source code is governed by MIT-like license that can be found in the
+ * LICENSE file in the root of the source tree. All contributing project authors
+ * may be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef ZLMEDIAKIT_MP3_H
+#define ZLMEDIAKIT_MP3_H
+
+#include "Extension/Frame.h"
+#include "Extension/Track.h"
+
+namespace mediakit {
+
+/**
+ * MP3音频通道
+ */
+class MP3Track : public AudioTrackImp{
+public:
+    using Ptr = std::shared_ptr<MP3Track>;
+    MP3Track(int sample_rate, int channels) : AudioTrackImp(CodecMP3,sample_rate,channels,16){}
+
+private:
+    Sdp::Ptr getSdp(uint8_t payload_type) const override;
+    Track::Ptr clone() const override;
+};
+
+}//namespace mediakit
+#endif //ZLMEDIAKIT_MP3_H