core: add trace API

Add a new API to collect traces from demuxer, decoder...

Instead of the previous log system, traces are stored under a JSON notation, it is useful to analyze them later.

Traces are structured as follows:

{
  "Timestamp": 11723487273,
  "Body": {
    "type": "DEC",
    "id": "audio/2",
    "stream": "IN",
    "pts": 4481452000,
    "dts": 4481452000
  }
}

It's inspired from: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/logs/data-model.md

Json logs may then be processed by a script to be displayed on a graph, here is the project link: https://gitlab.com/videolabs/public/vlc-pa

And here a render of what can be obtained: