Add support for image registry proxies

[radupopescu]

Provide the option to define a proxy for an input registry. Manifest and image download requests should be directed to the proxy for images belonging to the registry.

[radupopescu]

For the CVMFS unpacker to support the CERN image registry cache, a way to handling multi-arch image manifests is required.

• Cached images are stored as multi-arch images and provided with a manifest list (application/vnd.docker.distribution.manifest.list.v2+json).
• The CVMFS unpacker only supports simple manifests (application/vnd.docker.distribution.manifest.v2+json`).
• The CERN registry doesn't seem to offer backward compatibility, ignoring the Accept: application/vnd.docker.distribution.manifest.v2+json HTTP header).