增加http代理功能 (#2988)

支持通过http代理拉流, 避免内外网复杂网络环境时需要专门配置防火墙,现在只需要配置一台代理内网的服务器.

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Co-authored-by: xia-chu <771730766@qq.com>

src/Common/config.cpp

@@ -353,6 +353,7 @@ const string kBeatIntervalMS = "beat_interval_ms";
 const string kBenchmarkMode = "benchmark_mode";
 const string kWaitTrackReady = "wait_track_ready";
 const string kPlayTrack = "play_track";
+const string kProxyUrl = "proxy_url";
 } // namespace Client
 
 } // namespace mediakit