Draft: preparser: use of an external process for preparsing

This MR improves media parsing stability by isolating the preparser in a dedicated external process (vlc-preparser), using a JSON-based IPC protocol.

• External preparser process:

  • Creates vlc-preparser as a standalone process to handle parsing requests.
  • Uses JSON messages for IPC between VLC core and vlc-preparser process.
  • Read request on STDIN and print responce in STDOUT.
  • Send big attachments (picture_t or input_attachment_t) in base64 outside of the JSON.

• New preparser backend system:

  • Implements module-like interface for the preparser.
  • Maintains compatibility with existing preparser API.
  • Adds VLC_PREPARSER_BACKEND_PROCESS for external process execution and VLC_PREPARSER_BACKEND_THREADS for original implementation.
  • Updates playlist and the medialib thumbnailer to use the process backend.

• Serialization framework:

  • Implements JSON serialization/deserialization module for preparser_msg structure.

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Benchmark:

test on linux:

/!\ Missing test on windows and macos