Recursive json serialization for structured attributes (log.Any)

[ldemailly]

Recursive json serialization for structured attributes (log.Any) and also:

• Handles both errors with exported struct fields (as sub objects) and the ones who do not (calling .Error() to get the JSON string)
• Correctly handles nil as JSON null

Of note: performance is somewhat worse

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (52498c7) 100.00% compared to head (ac8c689) 100.00%.
Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            4         4           
  Lines          449       430   -19     
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- Hits           449       430   -19     

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