Add Longest Substring with K Distinct Characters sliding window algor…

[Subhamstar]

Summary

Added a hard sliding window algorithm: Longest Substring with K Distinct Characters.

Details

• Implemented sliding window + hashmap solution (O(n) time).
• Added JavaDoc and clean code following project guidelines.
• Added comprehensive JUnit tests to verify correctness and edge cases.
• Placed files in correct directory: com/thealgorithms/strings/

Why this is useful?

This is a commonly asked interview problem (Amazon, Google, Meta) and is not present in the repository. It demonstrates advanced sliding window patterns and educates beginners.