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 78.33%. Comparing base (8c8527c) to head (ee1092c).

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     78.32%   78.33%           
- Complexity     6651     6655    +4     
=========================================
  Files           750      751    +1     
  Lines         22143    22162   +19     
  Branches       4350     4353    +3     
=========================================
+ Hits          17344    17360   +16     
- Misses         4101     4103    +2     
- Partials        698      699    +1     

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[alxkm]

Looks good. Thank you for the contribution.