Push from OCI-SIF to OCI registries in native OCI tar layer format

[dtrudg]

SingularityCE 4.0 introduced the OCI-SIF format, where OCI images are embedded in a SIF, using OCI structures... albeit with a single SquashFS layer for the rootfs.

SingularityCE 4.1 allowed layers to be preserved, but converted to SquashFS, when an OCI image is pulled to an OCI-SIF.

We want to allow an OCI-SIF to be pushed to a registry, such that the image in the registry is in 'true' OCI format, with tar layers.

This would allow lossless round-tripping between SingularityCE's OCI-Mode / OCI-SIF and other OCI runtimes through an OCI registry.

On push with a non-default flag, the image should be converted back into tar layer format before or during the push process.

[dtrudg]

--layer-format tar as opposed to a default --layer-format squashfs ?

[dtrudg]

sylabs/oci-tools#53 & sylabs/oci-tools#54 have to be implemented to proceed with the Singularity specific code.