Initial OpenBSD support

[dteslya]

This PR follows hellt/vrnetlab#151 and adds OpenBSD support. I've included an example lab topology I used for testing and some basic documentation.

[codecov]

Codecov Report

Merging #1762 (bfa53d3) into main (46bd91f) will decrease coverage by 0.29%.
Report is 8 commits behind head on main.
The diff coverage is 3.50%.

❗ Current head bfa53d3 differs from pull request most recent head 2414123. Consider uploading reports for the commit 2414123 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1762      +/-   ##
==========================================
- Coverage   51.99%   51.71%   -0.29%     
==========================================
  Files         144      145       +1     
  Lines       13967    14024      +57     
==========================================
- Hits         7262     7252      -10     
- Misses       5894     5957      +63     
- Partials      811      815       +4     

Files	Coverage Δ
clab/register.go	100.00% <100.00%> (ø)
cmd/generate.go	65.47% <ø> (ø)
nodes/vr_openbsd/vr-openbsd.go	1.78% <1.78%> (ø)

... and 4 files with indirect coverage changes

[hellt]

Thanks @dteslya

[hellt]

The new release will have this change, but for the time being users who interested in this kind can download the binary with

sudo docker run --rm -v $(pwd):/workspace ghcr.io/oras-project/oras:v1.1.0 pull ghcr.io/srl-labs/clab-oci:24141233