Define datalad.containers-run.oci-runtime

[yarikoptic]

For

• Add skopeo-based adapter for working with OCI images #277

which ATM uses docker to run OCI images, we would want to add support and config variable such as

 [datalad "containers-run"]
        oci-runtime = apptainer

which would default to auto which would try apptainer, singularity, podman ... whatever we support in some order

note: I use containers-run since we use containers. as prefix for containers registry already, and ideally should not interfere