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
  • Flags in compiler activation vs. out-of-conda-forge usage conda-forge/conda-forge.github.io#2002
  • Unexpected -O2 flag when building on Debug with Conda stack-of-tasks/pinocchio#2449)

TODO:

• pixi.toml
• CI
• README.md
• lockfile update bot