PyPI: populate RichResult timestamp metadata

[lorepirri]

Populate RichResult.creation_time for PyPI releases using metadata already available in the existing PyPI JSON response.

A PyPI version may contain multiple distribution files, each with its own upload_time_iso_8601. This change uses the earliest available file upload timestamp as the release creation time.

No additional HTTP requests are introduced.

PyPI object	creation_time	revision_creation_time
Release/version	Earliest available upload_time_iso_8601 among its distribution files	None

The implementation preserves the current behavior for:

• yanked releases
• prerelease filtering
• invalid versions
• empty historical releases
• list options and final version selection

If a release has no files, or none of its files provides upload_time_iso_8601, creation_time remains None.

Tests cover:

• a normal single-file release
• releases with multiple files
• selection of the earliest timestamp independent of file ordering
• files with missing timestamps
• releases without an available timestamp
• existing live PyPI version-selection behavior

The PyPI documentation now includes the corresponding RichResult metadata table.