feat: retrieve a server or load balancer network attachment

[jooola]

Adds the (&Server{}).PrivateNetFor(&Network{}) and (&LoadBalancer{}).PrivateNetFor(&Network{}) methods to retrieve the private network attachment information.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.38%. Comparing base (9b89d96) to head (8aa5847).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #750      +/-   ##
==========================================
+ Coverage   78.27%   78.38%   +0.10%     
==========================================
  Files          55       55              
  Lines        5023     5039      +16     
==========================================
+ Hits         3932     3950      +18     
+ Misses        824      822       -2     
  Partials      267      267              

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[apricote]

Code looks fine 👍