Fix #141: Support for multi arch buildx images

[gfgf-brain]

Closes #141

Changes

• Implemented support for multi-architecture Docker images generated by buildx.
• The tool now correctly identifies and processes list.manifest.json files.
• Cleanup rules (ExcludeDockerImages, ExcludePath) now consider SHAs listed within manifest lists.
• Ensures that non-excluded image SHAs are preserved even when the manifest list is removed.

Testing

To verify this fix:

1. Build a multi-arch Docker image using buildx and push it to Artifactory.
2. Configure artifactory-cleanup to target this repository.
3. Use rules like DeleteDockerImagesOlderThan and ExcludeDockerImages to simulate a cleanup scenario where some architectures or tags should be kept.
4. Observe that the list.manifest.json is handled appropriately and that only the intended images (or manifest lists pointing to them) are removed, while non-excluded SHAs remain accessible.