Add Sudoku Solver using Backtracking algorithm - Implements depth-fir…

[harsha08-2k6]

• [x] I have read CONTRIBUTING.md.
• [x] This pull request is all my own work -- I have not plagiarized it.
• [x] All filenames are in PascalCase.
• [x] All functions and variable names follow Java naming conventions.
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• [x] All new code is formatted with clang-format -i --style=file path/to/your/file.java.