feat: load variants from files

[Hofer-Julian]

• Add support for variant files to Pixi
• Order (later means higher priority)
  • Default variants
  • Local variants.yaml for pixi-build-rattler-build
  • Variant files from the backend
  • Inline variants from the backend
• Most tests are in test: variant files pixi-build-testsuite#70
• Docs

Fixes: #4139

AI disclosure

• Docs were mostly generated by AI. I reviewed and adapted them

[ruben-arts]

Thanks!!