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/MP3Rtp.h

@@ -0,0 +1,57 @@
+/*
+ * 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_MP3RTP_H
+#define ZLMEDIAKIT_MP3RTP_H
+
+#include "Rtsp/RtpCodec.h"
+#include "Extension/Frame.h"
+#include "Extension/CommonRtp.h"
+
+namespace mediakit {
+
+class MP3RtpEncoder : public RtpCodec {
+public:
+    using Ptr = std::shared_ptr<MP3RtpEncoder>;
+
+    MP3RtpEncoder(int sample_rate = 44100, int channels = 2, int sample_bit = 16);
+
+    void outputRtp(const char *data, size_t len, size_t offset, bool mark, uint64_t stamp);
+
+    bool inputFrame(const Frame::Ptr &frame) override;
+
+private:
+    int _channels;
+    int _sample_rate;
+    int _sample_bit;
+};
+
+
+class MP3RtpDecoder : public RtpCodec {
+public:
+    using Ptr = std::shared_ptr<MP3RtpDecoder>;
+
+    MP3RtpDecoder();
+
+    bool inputRtp(const RtpPacket::Ptr &rtp, bool key_pos = false) override;
+
+private:
+    void obtainFrame();
+
+    void flushData();
+
+
+private:
+    uint64_t _last_dts = 0;
+    FrameImp::Ptr _frame;
+};
+
+}//namespace mediakit
+#endif //ZLMEDIAKIT_MP3RTP_H