Implement wheel_build_tag hook for unique wheel file names #1181

Summary

Implement the accepted proposal from PR #1066 (docs/proposals/wheel-build-tag-hook.md).
Tracking issue: #1059

Requirements

1. Global settings: Add a wheels section to the global settings file with a build_tag_hook field (ImportString via Pydantic) that points to a callable.

   wheels:
     build_tag_hook: "mypackage.hooks:build_tag_hook"

2. Hook signature: The configured callable receives ctx, req, version, and wheel_tags (all keyword-only) and returns Sequence[str] — ordered suffix segments like ["el9.6", "rocm7.1", "torch2.10.0"].

   def build_tag_hook(
       *,
       ctx: context.WorkContext,
       req: Requirement,
       version: Version,
       wheel_tags: frozenset[Tag],
   ) -> typing.Sequence[str]:
       ...

3. Suffix construction: Join returned segments with _ and append to the existing numeric build tag (e.g. 8 → 8_el9.6_rocm7.1).

4. Validation: Each segment must match [a-zA-Z0-9.] only. Fail the build on invalid characters or hook exceptions.

5. No-op default: When build_tag_hook is unset, no suffix is appended (current behavior preserved).

6. No wheel content access: The hook must not depend on wheel content, build env, or ELF info — it must produce the same result whether building fresh or retrieving from cache.

References

• Proposal: [proposal] wheel_build_tag hook for unique wheel file names #1066
• Original issue: feat: Unique wheel file names with a new wheel_build_tag hook #1059