Directory Support

[shizhMSFT]

This PR enables oras to push / pull a directory to / from a remote registry.

To push, the target directory is compressed into a tar.gz temproray file (on Linux, it sits at /tmp). Then the tar.gz file is pushed normally with media type application/vnd.oci.image.layer.v1.tar+gzip and extra annotations

• io.deis.oras.content.digest: the sha256 digest of the internal tar file. It is different from the digest of the tar.gz file.
• io.deis.oras.content.unpack: marked as true for extration when pull.

To pull, the tar.gz file is pulled to a temprorary directory. Then it is extracted and verified against io.deis.oras.content.digest.

Notes:

• Since the temp files are introduced, please call defer store.Close() for *FileStore to release resources.
• The pushed directory can be directly used as a layer of a container image without any modification.

Resolves #54

[jdolitsky]

/test

[shizhMSFT]

/test

[jdolitsky]

lgtm

[yuwaMSFT2]

LGTM