Add configuration support for allowing container images to be automatically pulled so that the user does not have to go and run add container image separately.

This is the behavior arguably most users expect to happen when an image does not exist on the system they are launching containers on.

container {
    name gortr {
        auto-pull
        image cloudflare/gortr
    }
}

The systemd unit file created for each container has podman run which would naturally do this, but it may be more desirable to have functionality where there is a check that the image specified by the user actually exists somehow before proceeding.

https://vyos.dev/T4487 is a related bug basically identifying that images aren't automatically pulled and must be manually added, this feature would address that.