Allow better control over wheel tags

https://github.com/pypackaging-native/pkgconf-pypi produces platform-specific but Python-independent tags. RIght now, I don't think there's any way to expose this.

Start by reading the linked pkgconf-pypi project to understand its platform-specific, Python-independent wheel tags, then inspect meson-python's wheel-tag generation entry points. Define how users should control these tags and verify that the resulting wheels expose the intended tags.

Written by the indexing model from the issue text.