log: Add String method to Value and KeyValue

[pellared]

Fixes #4952

Implementation based on: https://pkg.go.dev/log/slog#Value.String and https://pkg.go.dev/log/slog#Attr.String.

Comment that it is for debugging purpose only inspired by https://pkg.go.dev/time#Time.String

[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 83.8%. Comparing base (b7fdeb9) to head (664c409).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #5117     +/-   ##
=======================================
- Coverage   83.8%   83.8%   -0.1%     
=======================================
  Files        248     248             
  Lines      16253   16275     +22     
=======================================
+ Hits       13625   13643     +18     
- Misses      2340    2344      +4     
  Partials     288     288             

Files	Coverage Δ
log/keyvalue.go	96.7% <90.9%> (-1.0%)	⬇️

... and 1 file with indirect coverage changes

[MrAlias]

🚀