feat: add --untagged-only flag for independent untagged manifest cleanup

[balcsida]

Purpose of the PR

This PR introduces a new --untagged-only flag to the purge command, enabling users to clean up untagged manifests independently without affecting tagged manifests. This addresses the confusion and limitations reported in issues #480 and #486.

Changes

• Added --untagged-only flag that exclusively targets untagged manifests
• When --untagged-only is set, the --filter and --ago flags become optional
• Users can now clean up all untagged manifests across all repositories or within specific repositories
• Improved validation logic to prevent incompatible flag combinations

Usage Examples

# Delete all untagged manifests in all repositories
acr purge -r example --untagged-only

# Delete untagged manifests in specific repository
acr purge -r example --filter "hello-world:.*" --untagged-only

Breaking Changes
None. The existing --untagged flag behavior remains unchanged for backward compatibility.

Fixes #480 and #486