[Pixi][CI] Add pixi support

[olivier-roussel]

Adds Pixi support for developers and CI.
Enables to:

• Have a standard way for a new developer to configure/build the project (homogeneous with other projects) and available on all platforms
• All dependencies stored in one file
• Have an easy way to enable some optional features
• Have a lock file for a reproducible build in CI

TODO:

• base pixi.toml file with build scopes
• features based on CMake presets (see [CMake] Modify presets to generalize its use #5272)
• lockfile update bot (require Github app https://github.com/apps/update-pixi-lockfile ) To be done in a dedicated PR
• pixi-based CI for linux-64 / win-64 / osx-64 / osx-arm64
• pixi-based CI for linux-arm64 Delayed after SOFA-GLFW / Imgui conda packaging

More info on pixi: https://pixi.sh/

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By submitting this pull request, I acknowledge that
I have read, understand, and agree SOFA Developer Certificate of Origin (DCO).

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[olivier-roussel]

Requires #5526 and sofa-framework/CGALPlugin#33

[alxbilger]

I just tested pixi run --environment full-dev runSofa and it works properly without almost any dependency installed on my system (Ubuntu 25.10).