Add Baby-Step Giant-Step (BSGS) Discrete Logarithm Algorithm

[rajucreate]

Summary

This PR adds the Baby-Step Giant-Step (BSGS) algorithm for solving the discrete logarithm problem:

Find x such that: a^x ≡ b (mod m)

This algorithm is widely used in number theory and cryptography and runs in O(√m) time.

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What’s Included

i) Added full implementation:

• src/main/java/com/thealgorithms/maths/DiscreteLogarithmBSGS.java

ii) Added JUnit tests:

• src/test/java/com/thealgorithms/maths/DiscreteLogarithmBSGSTest.java

iii) Added entry in DIRECTORY.md under the maths section

iv) Includes detailed Javadoc and follows repository coding conventions

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Algorithm Overview

• Time Complexity: O(√m)
• Space Complexity: O(√m)
• Uses the Baby-Step Giant-Step technique to compute discrete logarithms efficiently
• Includes modular exponentiation helper using fast power

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• [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.
• [x] 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