Allow referencing topologies from http(s) locations

[hellt]

In addition to be able to provide git URLs for clab deploy command this PR adds ability for clab to handle HTTP(S) URLs, such as:

clab dep -c -t https://gist.githubusercontent.com/hellt/66a5d8fca7bf526b46adae9008a5e04b/raw/034a542c3fbb17333afd20e6e7d21869fee6aeb5/linux.clab.yml

This feature is tailored to the self-container labs which has everything they need in the topo file itself. Since such files can be located outside of git repositories we add handling of HTTP(s) locations.

for example this is now the way to quickly start a single srl node:

clab dep -c -t srlinux.dev/clab-srl

[codecov]

Codecov Report

Merging #1704 (cefa240) into main (b388c3e) will increase coverage by 0.11%.
The diff coverage is 77.16%.

❗ Current head cefa240 differs from pull request most recent head eef2a49. Consider uploading reports for the commit eef2a49 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1704      +/-   ##
==========================================
+ Coverage   51.56%   51.68%   +0.11%     
==========================================
  Files         141      141              
  Lines       13640    13718      +78     
==========================================
+ Hits         7034     7090      +56     
- Misses       5839     5854      +15     
- Partials      767      774       +7     

Files	Coverage Δ
clab/config.go	66.83% <100.00%> (ø)
git/github.go	92.75% <100.00%> (+1.22%)	⬆️
git/repo.go	80.64% <100.00%> (+4.64%)	⬆️
nodes/srl/srl.go	57.19% <0.00%> (ø)
clab/clab.go	76.59% <83.33%> (+0.09%)	⬆️
utils/file.go	55.79% <80.39%> (+3.03%)	⬆️
cmd/root.go	76.69% <62.50%> (-3.31%)	⬇️

... and 2 files with indirect coverage changes