Added ElGamal Encryption Algorithm

[Shewale41]

• I have read CONTRIBUTING.md
• Code follows TheAlgorithms/Java package structure and naming conventions
• SpotBugs, Checkstyle, and CodeQL pass
• Javadoc comments added for all public methods and classes
• Added JUnit test cases for validation
• Verified encryption-decryption correctness
• Code formatted with clang-format

[codecov-commenter]

Codecov Report

❌ Patch coverage is 91.56627% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.40%. Comparing base (21eff8a) to head (f646e48).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
...ithms/datastructures/trees/ThreadedBinaryTree.java	90.19%	2 Missing and 3 partials ⚠️
...a/com/thealgorithms/ciphers/ElGamalEncryption.java	90.00%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7004      +/-   ##
============================================
+ Coverage     78.26%   78.40%   +0.13%     
- Complexity     6573     6641      +68     
============================================
  Files           743      750       +7     
  Lines         21895    22098     +203     
  Branches       4288     4331      +43     
============================================
+ Hits          17137    17326     +189     
- Misses         4074     4082       +8     
- Partials        684      690       +6     

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