Add extended euclidean algorithm

[t-summ]

PR Checklist:

• [X] My submission is formatted according to the guidelines in the contributing guide
• [ ] My addition is on refer on the language README.md file
• [X] My addition does not have a spelling problem
• [X] My submission has a proper and user-friendly description of the algorithm
• [ ] My submission has the time complexity of the algorithm
• [X] My submission has sample input-output of the program (NOT FOR PYTHON)

What kind of change does this PR introduce? (check at least one)

• [ ] Bugfix
• [X] New algorithm
• [ ] Optimization in previous algorithms
• [ ] Code style update
• [ ] Refactor
• [ ] Documentation
• [ ] Other, please describe:

Briefly describe the changes in this PR

This pull request adds the Extended Euclidean Algorithm to the Maths algorithm of the CPlusPlus directory.

Other information:

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