Add 4-Sum Problem implementation

[volimroze]

Description:

This PR adds an implementation of the 4-Sum Problem in Java. It uses a two-pointer approach to find all unique quadruplets that sum up to a target value.

• The class FourSumProblem is added under the package foursum.

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work.
• 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.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 24 lines in your changes missing coverage. Please review.

Project coverage is 64.12%. Comparing base (c301fec) to head (551a7ce).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...in/java/com/thealgorithms/misc/FourSumProblem.java	0.00%	24 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5787      +/-   ##
============================================
- Coverage     64.13%   64.12%   -0.02%     
- Complexity     4205     4217      +12     
============================================
  Files           587      589       +2     
  Lines         16388    16450      +62     
  Branches       3163     3180      +17     
============================================
+ Hits          10511    10549      +38     
- Misses         5448     5472      +24     
  Partials        429      429              

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[siriak]

We don't add leetcode problems