overlay seal and conversion in singularity push of OCI-SIF as tar layers

[dtrudg]

When an OCI-SIF image is push-ed to an OCI registry, and it is requested to use the tar layer format (#2848) then overlays must be handled as part of the push process:

• An extfs overlay must be sealed (singularity overlay seal conversion of overlay to r/o layer in OCI-SIF #2870) into a read-only layer.
• The layer must be pushed in tar format.