I want to add C++ code of AVL Tree

#### This issue is about [AVL Tree].

- [X] Issue Title is in form `avl_tree [C++]`.
- [X] I searched or browsed the repo’s other issues to ensure this is not a duplicate.
- [X] This Algo/DS is worth adding to this repository.
- [X] Assign this issue to me (I want to work on this).
- [X] I ensure that I am not already assigned to 2 or more issues.
- [X] I will send my PR only after I'm assigned to this issue by any maintainer.
- [X] I understand that I will be unassigned from this issue if I fail to send a PR for this within a week.

Start by browsing the repository’s existing data-structure implementations and C++ examples to find the expected location, naming, and style for an AVL tree. Add the C++ AVL tree implementation in the matching location and verify it follows the repository’s conventions; no specific test or entry point is named in the issue.

Written by the indexing model from the issue text.