added feature BST & AVL

[amarmaurya-com]

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 288 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.33%. Comparing base (bb6385e) to head (f137558).

Files with missing lines	Patch %	Lines
src/main/java/com/thealgorithms/tree/AVL.java	0.00%	156 Missing ⚠️
src/main/java/com/thealgorithms/tree/BST.java	0.00%	132 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7009      +/-   ##
============================================
- Coverage     78.35%   77.33%   -1.02%     
  Complexity     6615     6615              
============================================
  Files           746      748       +2     
  Lines         22015    22303     +288     
  Branches       4320     4347      +27     
============================================
  Hits          17249    17249              
- Misses         4078     4366     +288     
  Partials        688      688              

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