Add pixi support

[jorisv]

This PR add Pixi support for developer and CI.

This aims to have:

• A standard way to new developer to configure/build the project
• All dependencies stored in one file
• A way to easily know optional dependencies (hpp-fcl to use the COLLISION_SUPPORT option)
• A lock file to have a more reproducible build in CI
• Overcome the conda cxx-compiler package CXXFLAGS issue
  • https://github.com/conda-forge/conda-forge.github.io/issues/2002
  • https://github.com/stack-of-tasks/pinocchio/issues/2449)

TODO:

• [x] pixi.toml
• [ ] CI
• [ ] README.md
• [ ] lockfile update bot