Pre-imports failing due to unsupported SIF media types

Context

Found during gitlab#350920 (closed).

Problem

A pre import failed due to:

pre importing tagged manifests: pre importing manifest: pre importing manifest: unknown media type: application/vnd.sylabs.sif.config.v1+json

Solution

Add support for application/vnd.sylabs.sif.config.v1+json and while at it also application/vnd.sylabs.sif.layer.v1.sif

• https://sylabs.io/guides/3.4/user-guide/singularity_and_docker.html
• https://github.com/apptainer/singularity/issues/4437