add extfs overlay to OCI-SIF via `singularity overlay`

[dtrudg]

The singularity overlay create command should be extended to add an extfs overlay to an OCI-SIF image, in a similar way as is done for a native mode SIF.

The overlay added to the OCI-SIF should be included in an updated image manifest / config, with content-hashes that are consistent for the initial state of the overlay.