Epic: Prototype package build

[msarahan]

Designs that need to be proven out (edit this issue to add links to PRs or repos):

• How do projects specify that they use data from a metadata provider at package build time?
  • New pyproject.toml project key: add readme and draft pyproject.toml test-data-creation#1
• How do package build tools (e.g. scikit-build-core) interpret package input data? How do they act on it?
• How do filenames of packages with different build metadata differ? They must differ somehow, or else variants would clobber one another
  • get extra identifying string from metadata provider
  • hash variant key/values
  • Where to put extra "bits"?
    • add to build tag
    • add to platform tag