Add support for podman volume export and import in libpod api

[Riushda]

Feature request description

I would like to be able to export podman containers named volumes using libpod api via unix podman socket. I would use it to make regular backup of my volumes. The import command would be also a nice addition. I searched in documentation and in the code and I don't think it is possible to do it using the api yet.

Suggest potential solution

I think the feature is somehow already present in the exportcheckpoint function: https://github.com/containers/podman/blob/main/libpod/container_internal_common.go#L1205. However doing a checkpoint requires to stop the container and save many things from the container I don't need (I only need the volumes). I'm not a go developer so I apologize if this part of the code is irrelevant.

Have you considered any alternatives?

Yes the create checkpoint endpoint but it is service interrupting and backup more than just the volumes.

[mheon]

Seems perfectly reasonable.