Store artifact image as oci-dir

[termoshtt]

• image::{Artifact, Builder} are introduced to represent ocipkg's artifact and its builder.
  • This supports application/vnd.ocipkg.v1.artifact and old style artifact used in 0.2.x. It is called v0 format.
• The manifest and blobs stored in local storage by #107 has been changed also in this PR. They are stored as usual oci-dir format:

.
├── libocipkg_static_rust.a
└── .oci-dir
    ├── blobs
    │   └── sha256
    │       ├── 7299ae02d5a3ac14e45c62670341d869f4cb3b41eebbfa59acbcfdc6e864288a
    │       ├── b9a51607f9fbe50601a623cf5060506dee43798db9e1ccc014404edffd2b5c2b
    │       └── 2aedb7a0fd572b2cd4dd3408f7f83642f76ced60308db082470c76ed52d3845c
    └── index.json