get license file from a remote URL

[hellt]

Discussed in https://github.com/srl-labs/containerlab/discussions/1376

^{Originally posted by jcpvdm May 12, 2023} For example:

topology:
  kinds:
    vr-sros:
      image: registry.srlinux.dev/pub/vr-sros:23.3R2
      type: sr-1
      license: ftp://user:[email protected]/licenses.txt
```</div>