Add style guide for manual getters on struct typed storage variables

[maurelian]

Adds a style guide documenting the ideal pattern for struct typed storage variables.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.27%. Comparing base (40a1a99) to head (32d6532).
⚠️ Report is 6 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #18345      +/-   ##
===========================================
- Coverage    76.58%   76.27%   -0.31%     
===========================================
  Files          179      179              
  Lines        10732    10732              
===========================================
- Hits          8219     8186      -33     
- Misses        2367     2402      +35     
+ Partials       146      144       -2     

Flag	Coverage Δ
cannon-go-tests-64	66.58% <ø> (-0.82%)	⬇️
contracts-bedrock-tests	82.10% <ø> (ø)

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