create a firewall interface

[steiler]

This is creating a firewall interface and reintroduce the iptables integration as a fallback.

[steiler]

This is ready for you to review / merge now @hellt.

[hellt]

Thanks @steiler
LGTM, just added some comments to exported funcs

[codecov]

Codecov Report

Attention: 147 lines in your changes are missing coverage. Please review.

Comparison is base (30dfa30) 51.82% compared to head (addc92b) 51.59%.
Report is 8 commits behind head on main.

❗ Current head addc92b differs from pull request most recent head 6abd0e7. Consider uploading reports for the commit 6abd0e7 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1850      +/-   ##
==========================================
- Coverage   51.82%   51.59%   -0.23%     
==========================================
  Files         148      152       +4     
  Lines       14548    14634      +86     
==========================================
+ Hits         7539     7550      +11     
- Misses       6176     6249      +73     
- Partials      833      835       +2     

Files	Coverage Δ
runtime/docker/firewall/firewall.go	54.54% <54.54%> (ø)
runtime/docker/firewall/nftables/rule.go	83.01% <83.01%> (ø)
runtime/docker/firewall.go	33.33% <33.33%> (ø)
runtime/docker/firewall/nftables/client.go	70.73% <70.73%> (ø)
runtime/docker/firewall/iptables/client.go	0.00% <0.00%> (ø)

... and 2 files with indirect coverage changes