libimage.Image: add ConvertToManifestList()

[nalind]

• Add libimage.Image.ConvertToManifestList(), which will convert an image to a manifest list if it isn't already possible to use it as one.
• Tweak the godoc for the libimage/manifests.AddArtifactOptions type so that it gets wrapped correctly for display.

[packit-as-a-service[bot]]

We were not able to find or create Copr project packit/containers-common-2091 specified in the config with the following error:

Packit received HTTP 500 Internal Server Error from Copr Service. Check the Copr status page: https://copr.fedorainfracloud.org/status/stats/, or ask for help in Fedora Build System matrix channel https://matrix.to/#/#buildsys:fedoraproject.org.

---

Unless the HTTP status code above is >= 500, please check your configuration for:

1. typos in owner and project name (groups need to be prefixed with @)
2. whether the project name doesn't contain not allowed characters (only letters, digits, underscores, dashes and dots must be used)
3. whether the project itself exists (Packit creates projects only in its own namespace)
4. whether Packit is allowed to build in your Copr project
5. whether your Copr project/group is not private

[rhatdan]

LGTM @giuseppe @mtrmac PTAL

[nalind]

Updated to set preferredListType correctly for Docker manifest types, and to drop tag names from the temporary OCI layouts the code creates.

[rhatdan]

@mtrmac PTANL

[rhatdan]

@mtrmac @giuseppe PTANL

[rhatdan]

@mtrmac @giuseppe PTANL

[rhatdan]

@mtrmac @giuseppe PTANL

[rhatdan]

/approve /lgtm

[openshift-ci[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: nalind, rhatdan

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