Make backends agnostic to toml version

[tdejager]

It would be great if the backends are agnostic to the pixi.toml version as this is causing a lot of problems in CI and also in the future. This is mainly caused by the build backends reading the pixi.toml, this means that if we add a field in pixi_manifest the backends cannot handle this, until we update the backends. This causes a chicken and egg problem.

Solving this requires extending the protocol and disallowing the backends to read the pixi.toml files.