Store Manifest JSON and blobs

[termoshtt]

Changes

• ocipkg get returns error if the container is already downloaded. ocipkg get --overwrite option is added to remove existing.
• ocipkg get store image manifest as .manifest.json and raw blobs where the contents are stored. For example, ocipkg get ghcr.io/termoshtt/ocipkg/static/rust:fb58644 yields following directory. This helps managing non-tar.gz container including OCI Artifact manifest #78 and Restore oci-archive from locally stored files #44

.
├── .blob
│   └── sha256
│       └── df643e35140fb7ee1c4703b5b14ae10ab9aa7971af0a9bcf8f1212c44a14f270
├── .manifest.json
└── libocipkg_static_rust.a

[termoshtt]

This has been changed by #108