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.