feat: Add OCI Image Configuration structures

[Xynnn007]

Related to https://github.com/krustlet/oci-distribution/issues/50, added structure definitions.

[Xynnn007]

How do you intend for this to be consumed? Does the user use the config type if they want it and then have to serialize it to Vec<u8>? Or is there another way to make it easier? Just want to make sure I understand!

I've added a new API oci_v1_from_config_file to construct a Config using a ConfigFile. Typical scenarios:

• A user wants to push a cosign signature and create a related structured Config
• A user wants to pull an image using pull() to get ImageData, and then he can parse the image_data.config.data into a structure to do some other things.

To me the first scenario is needed, because current impl uses an empty config, which may be detected as illegal image for some registries. Thus, I want to provide some detailed support for OCI Configuration and let users use it arbitrarily.

For some configurations inside a registry may seem different from what the spec defines, I did not integrate deserialization of config into fn pull and leave it to users to use by their own and to their needs.