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 #2449)

Bonus:

• Fix cppad and cppadcg build with last cppad version.
• Fix mjcf unittest fail with last boost version
• Fix sincos test fail on OSX Debug
• Fix constraint-variants test fail on Linux (uninitialized values)
• Fix mixing binding symbol issue on OSX Debug (Set -fvisibility=hidden on OSX and GNU/Linux #2462)

TODO:

• pixi.toml
• debug cppad/cppadcg finder
• CI
• README.md
• lockfile update bot

[github-actions]

👋 Hi,
This is a reminder message to assign an extra build label to this Pull Request if needed.
By default, this PR will be build with minimal build options (URDF support and Python bindings)
The possible extra labels are:

• build_collision (build Pinocchio with coal support)
• build_casadi (build Pinocchio with CasADi support)
• build_autodiff (build Pinocchio with CppAD support)
• build_codegen (build Pinocchio with CppADCodeGen support)
• build_extra (build Pinocchio with extra algorithms)
• build_mpfr (build Pinocchio with Boost.Multiprecision support)
• build_sdf (build Pinocchio with SDF parser)
• build_accelerate (build Pinocchio with APPLE Accelerate framework support)
• build_all (build Pinocchio with ALL the options stated above)

Thanks.
The Pinocchio development team.