Add boundary traversal of binary tree

[xuyang471]

This PR implements the boundary traversal algorithm for a binary tree as requested in the related issue. The boundary traversal is performed in three parts:

Left Boundary: Traverses the left boundary of the tree, excluding leaf nodes.
Leaf Nodes: Collects all leaf nodes from left to right.
Right Boundary: Traverses the right boundary of the tree, excluding leaf nodes, and adds the nodes in reverse order to ensure correct boundary order.
Both time complexity and space complexity of this implementation are O(n), where n is the number of nodes in the binary tree.

The boundary traversal is performed both iteratively and recursively, and edge cases such as single-node trees and trees without left or right subtrees are handled appropriately.
All methods in the test class passed the test.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 59.58%. Comparing base (f170078) to head (46bdd25).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5639      +/-   ##
============================================
+ Coverage     59.42%   59.58%   +0.16%     
- Complexity     3808     3847      +39     
============================================
  Files           558      559       +1     
  Lines         15949    16015      +66     
  Branches       3048     3073      +25     
============================================
+ Hits           9477     9543      +66     
  Misses         6069     6069              
  Partials        403      403              

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[siriak]

Looks good, thanks!