Added FourSum Problem

[shivam-1810]

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.05%. Comparing base (3af4cfd) to head (f327e75).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5830      +/-   ##
============================================
+ Coverage     65.96%   66.05%   +0.09%     
- Complexity     4386     4418      +32     
============================================
  Files           602      603       +1     
  Lines         16783    16825      +42     
  Branches       3226     3244      +18     
============================================
+ Hits          11071    11114      +43     
  Misses         5266     5266              
+ Partials        446      445       -1     

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