Add SRT Support

[yegors]

This update introduces a new SRT output feature that allows RTLSDR‑Airband to stream audio using the Secure Reliable Transport protocol.

• Build support – SRT requires the libsrt development files; CMake will enable it automatically when the library is available
• Audio formats – Streams can be sent as raw PCM, MP3, or WAV depending on the format setting
• Low latency playback – When using mp3 or wav, clients such as ffplay can connect with minimal delay using: ffplay -fflags nobuffer -flags low_delay srt://<host>:<port>

Configuration example

outputs: (
  {
    type = "srt";
    listen_address = "0.0.0.0";
    listen_port = 8890;
    format = "mp3";       # pcm|mp3|wav
    continuous = true;    # optional, default false
  }
);

I'm not a C++ dev, and this code has been entirely AI written so definitely needs some human eyes on it. But it does work.

Resolves #392

[yegors]

Been running this server for ~2 weeks now, appears stable. I'm using it here: https://github.com/yegors/co-atc