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

[charlie-foxtrot]

@yegors thanks for the PR and sorry for the delay. It looks like there are a few things in here that could be pulled out into independent smaller PRs and get in sooner (addition of min_rx_seconds, changes to UDP streams). For the core of this PR, I'm going to some housekeeping work on the repo before landing the larger changes.

[yegors]

Yeah, I mostly just kept committing to my branch for changes I needed along the way for my project to make it output clean streams I can feed directly into TTS with no encoding. Feel free to copy whatever you want.