Add custom mapper for snake case to camel case #831

[ajozwik]

Implementation for mapper for snake case to camel case described in #831

[codecov]

Codecov Report

❌ Patch coverage is 90.58824% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.14%. Comparing base (052df3e) to head (29fda34).
⚠️ Report is 105 commits behind head on master.

Files with missing lines	Patch %	Lines
...ime/derivation/transformer/TransformerMacros.scala	81.81%	4 Missing ⚠️
.../compiletime/dsl/TransformerIntoForAllMacros.scala	0.00%	2 Missing ⚠️
...nd/chimney/integrations/FactoryCompatBuilder.scala	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #879      +/-   ##
==========================================
+ Coverage   83.71%   84.14%   +0.43%     
==========================================
  Files         170      171       +1     
  Lines        7351     7450      +99     
  Branches      816      791      -25     
==========================================
+ Hits         6154     6269     +115     
+ Misses       1197     1181      -16     

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