tools: Export package metadata and RGB565 icons.

[aviralgarg05]

Note: Please adhere to Contributing Guidelines.

Summary

tools/export_pkg_repo.py produced a catalog with a name, a version and an
artifact, which is enough to install a package but not enough for a client
to show anything about it before installing.

This lets a package carry a description, a category and an icon. The icon
is converted from an ordinary image into a small raw RGB565 buffer with a
12-byte header, which a target can render directly; decoding PNG or JPEG on
a small device is not assumed.

Impact

• New feature: YES, optional per-package metadata and icons.
• User adaptation: NO. The extra fields are optional, and a catalog without
  them exports exactly as before.
• Build: NO, host tool only.
• Hardware: NO.
• Documentation: The repository format is described in Documentation: Update nxpkg and add nxstore guide. nuttx#18875.
• Security: NO. Icons are converted locally, not fetched.
• Compatibility: The added fields are ignored by a client that does not
  know them.

Testing

Build host: macOS 26.5, arm64, Python 3.14.

• flake8 with .github/linters/setup.cfg, black --check, isort --check
• exported a repository with and without icons and confirmed index.json
  parses, the SHA-256 of each artifact matches the file on disk, and the
  icon payload is the expected size for its declared dimensions

PR verification Self-Check

• This PR introduces one focused change.
• I have updated all required description fields above.
• I have reviewed and signed every commit.
• This PR adheres to the current contribution and coding guidelines.
• My PR is still work in progress.
• My PR is ready for review and can be safely merged.