Add ability to set artifactType at the image index level

[ipanova]

Feature request description

Based on the OCI image-spec image index can contain artifactType field. . With the current podman version (5.1.2) it is possible to set this field only for child manifest via podman manifest add --artifact-type

Suggest potential solution

Add ability to set artifactType field at the image index level.
For example at creation step podman manifest create --artifact-type

Have you considered any alternatives?

A clear and concise description of any alternative solutions or features you've considered.

Additional context

Add any other context or screenshots about the feature request here.

[mheon]

We are adding a dedicated set of artifact-handling commands. This should be part of them. @baude FYI

[github-actions]

A friendly reminder that this issue had no activity for 30 days.