Add ElGamalCipher implementing ElGamal encryption and decryption

[polasisubash]

Description

Added a new class ElGamalCipher.java implementing the ElGamal encryption and decryption algorithm.

Features

• Implements ElGamal public-key encryption system
• Supports key generation using modular arithmetic
• Includes methods for encryption and decryption
• Demonstrates successful decryption of encrypted messages

Example Output

Original: 12345 Encrypted: (c1, c2 values) Decrypted: 12345

Technical Notes

• Implemented under src/main/java/com/thealgorithms/ciphers/
• Successfully compiled and tested locally using Maven (BUILD SUCCESS)
• Compatible with Java 21
• Code adheres to the project’s structure and conventions

References

• Fixes issue: #13580
• Algorithm based on ElGamal public-key cryptography principles

Checklist

• [x] Code compiles successfully
• [x] Tests passed locally (BUILD SUCCESS)
• [x] Follows project style and structure
• [x] Added clear example output
• [x] Confirmed correct encryption and decryption results