add CombinationSum and test

[Avaneeshakrishna]

• 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

[Avaneeshakrishna]

Pls add Hactoberfest label tag to this PR

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.94737% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.61%. Comparing base (c691b31) to head (3ff2e3b).

Files with missing lines	Patch %	Lines
...com/thealgorithms/backtracking/CombinationSum.java	78.94%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #6725      +/-   ##
============================================
+ Coverage     76.60%   76.61%   +0.01%     
- Complexity     6026     6031       +5     
============================================
  Files           719      720       +1     
  Lines         20326    20345      +19     
  Branches       3937     3940       +3     
============================================
+ Hits          15571    15588      +17     
- Misses         4164     4166       +2     
  Partials        591      591              

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