images: inspect remote registry in ImagesManager.get_registry_data (Fixes #665)

[akshat-lakhera]

Fixes #665 - ImagesManager.get_registry_data() only inspects the local image.

ImagesManager.get_registry_data previously called self.get(name), which only inspected local image storage and failed with ImageNotFound when an image was not already pulled. It also ignored auth_config credentials.

Summary of changes:

• Updated ImagesManager.get_registry_data to query Libpod GET /manifests/{name}/json to inspect the remote manifest directly without pulling the image.
• Added auth_config support across ManifestsManager.get and ImagesManager.get_registry_data via base64 encoded X-Registry-Auth headers.
• Captured Docker-Content-Digest response headers to populate the digest attribute.
• Expanded RegistryData.id to resolve digests from Id, Descriptor, multi-arch manifests matching host architecture, and config.
• Enhanced RegistryData.has_platform to check multi-arch manifests and platform descriptors.
• Updated ImagesManager.pull to format digest references with repository@sha256:... notation.
• Added RegistryData.reload to refresh metadata directly from the registry.
• Added unit tests in test_imagesmanager.py, test_registrydata.py, and test_manifests.py.

Signed-off-by: Akshat Lakhera akshatlnct2024@gmail.com