feat: support variant files

[Hofer-Julian]

• Add support for variant files to the backend
• 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
• There are a couple of tests to verify this behavior, others are in test: variant files pixi-build-testsuite#70

Also updated rattler and Pixi