Add AVL Tree data structure implementation

[nadunhasalanka]

This pull request adds a complete AVL Tree implementation to the data structures package.

• Added AVLTree.java file in the src/datastructures/avltree/ directory
• Implemented insert, delete, and search operations with automatic tree balancing
• Created all four rotation methods to maintain AVL tree balance properties
• Added tree traversal methods including in-order, pre-order, post-order, and level-order
• Included a tree validation method to verify AVL properties are maintained
• Created a visual display method to show tree structure level by level
• Added testings in the main method covering all operations

[github-actions]

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