get license file from a remote URL

### Discussed in https://github.com/srl-labs/containerlab/discussions/1376

<div type='discussions-op-text'>

<sup>Originally posted by **jcpvdm** May 12, 2023</sup>
For example:

```
topology:
  kinds:
    vr-sros:
      image: registry.srlinux.dev/pub/vr-sros:23.3R2
      type: sr-1
      license: ftp://user:pass@1.2.3.4/licenses.txt
```</div>