Refactor GHContentSearchBuilder to allow a Fork type

[sahansera]

Description

• Introduce a new enum GHFork.
• Refactor the existing code to use the new GHFork enum.
• Add a new unit test to test GitHub code search including results of forks.

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[codecov]

Codecov Report

Merging #1315 (4828993) into main (3069134) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##               main    #1315      +/-   ##
============================================
+ Coverage     77.92%   78.02%   +0.09%     
- Complexity     2016     2027      +11     
============================================
  Files           198      199       +1     
  Lines          6224     6243      +19     
  Branches        355      356       +1     
============================================
+ Hits           4850     4871      +21     
+ Misses         1164     1162       -2     
  Partials        210      210              

Impacted Files	Coverage Δ
...ava/org/kohsuke/github/GHContentSearchBuilder.java	80.76% <100.00%> (+6.85%)	⬆️
src/main/java/org/kohsuke/github/GHFork.java	100.00% <100.00%> (ø)
.../org/kohsuke/github/GHRepositorySearchBuilder.java	81.57% <100.00%> (+2.63%)	⬆️
.../main/java/org/kohsuke/github/GHSearchBuilder.java	100.00% <100.00%> (ø)

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[bitwiseman]

Generally a good change. Need adjustment maintain backward compatibility.