oci: support for SCIF, incl. --app

[preminger]

Description of the Pull Request (PR):

Adds support for SCIF apps in OCI-mode using --app command-line option.

NOTE: This feature assumes a Docker-style SCIF container built with the scif executable as CMD or ENTRYPOINT, as per the instructions in the SCIF tutorial (archived here).

This fixes or addresses the following GitHub issues:

• Fixes Support --app in --oci mode (SCIF in OCI) #1470

[dtrudg]

A couple of comments left.

More generally, I don't think that having inspect --oci dump out SCIF specific output is right.

• I think inspect --oci --app xxx output is fine to show SCIF app xxxx
• I think that inspect --oci --list-apps showing all SCIF apps would be fine.
• I think that a general inspect --oci should show OCI config for any OCI-SIF image... but that is outside of the scope here.

It's probably best to pull out the inspect stuff here, and then...

1. Implement the general OCI-SIF inspect.
2. Add the SCIF specific piece.

[preminger]

Tentative implementation of singularity inspect --oci for Docker-style SCIF containers has been dropped from this PR, and can be found here: #2360

[vsoch]

Just saw this in my Twitter DMs - sorry I missed the earlier discussion. Super cool and nice job @preminger !