Add PEP 794 import name metadata support

[Chris79OG]

Summary

• accept static project.import-names and project.import-namespaces values from pyproject.toml
• emit Import-Name / Import-Namespace Core Metadata 2.5 headers and read them back from PKG-INFO
• preserve existing PEP 794 validation for duplicate names and missing parents

Closes #5112.

Testing

• python -m pytest setuptools\tests\config\test_pyprojecttoml.py setuptools\tests\test_core_metadata.py setuptools\tests\config\test_apply_pyprojecttoml.py -q
• git diff --check (passes with existing LF-to-CRLF warnings on Windows)

Checklist

• News fragment added in newsfragments/.