customizable client ip extractor

[billyplus]

For who need to customize his own client IP extractor.

Fixes #2474 and #2232

[codecov]

Codecov Report

Merging #2608 (023eb48) into master (4d2dad5) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2608   +/-   ##
=======================================
  Coverage   98.49%   98.49%           
=======================================
  Files          41       41           
  Lines        1989     1991    +2     
=======================================
+ Hits         1959     1961    +2     
  Misses         17       17           
  Partials       13       13           

Impacted Files	Coverage Δ
gin.go	99.00% <ø> (ø)
context.go	97.52% <100.00%> (+0.01%)	⬆️

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