feat: Add Hierholzer's Algorithm for Eulerian Circuits (#6726)

* feat: Add Hierholzer's Algorithm for Eulerian Circuits

* fix: Add more test cases to improve code coverage

* feat: Add Hierholzer's Algorithm for Eulerian Circuits

* fix: Apply clang-format after merge

* fix: Apply all formatting, style, and efficiency fixes

* docs: Apply feedback and improve Javadoc

* docs: Add Hierholzer's Algorithm to DIRECTORY.md