Added support for customizing generated output

[gmazzo]

Addresses #178 by adding support for extending the default Kotlin and Java generator's output

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 10 lines in your changes missing coverage. Please review.

Project coverage is 15.07%. Comparing base (e43b032) to head (7a8f6a8).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...buildconfig/generators/BuildConfigJavaGenerator.kt	0.00%	5 Missing ⚠️
...ildconfig/generators/BuildConfigKotlinGenerator.kt	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #181      +/-   ##
==========================================
- Coverage   15.28%   15.07%   -0.21%     
==========================================
  Files          19       19              
  Lines         589      597       +8     
  Branches      163      163              
==========================================
  Hits           90       90              
- Misses        482      490       +8     
  Partials       17       17              

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