pixi based (+ dev container) workspace configuration for phoenix projects

To have a uniform development experience for all phoenix packages regardless of programming language, we will implement a workflow based on pixi and/or dev containers:

• figure out a pixi.toml configuration that
  • allows to use development containers
  • allows to have test environment already available in container?
    • downloading container may not be much faster than downloading packages but
      • but CI runner have cache for docker containers
      • containers are stored in gitlab -> working when gitlab is working, potentially use of internal network
• Use PhoenixCMakePixi to store the package level information in a single place (pixi.toml) and populate CMakeLists and then rattler-build recipe
• Use workflow in all phoenix2 repositories

FYI @paubert @thibaut.oprinsen