feat: optimal binary search tree added to dynamic_programming

[butterpaneermasala]

Description of Change

Added the implementation of the Optimal Binary Search Tree (OBST) algorithm in C++ under the dynamic_programming namespace. The OBST algorithm constructs a binary search tree that minimizes the expected search cost given a set of keys and their search probabilities.

This implementation follows a dynamic programming approach with a time complexity of O(n³).

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes: Added optimal_bst.cpp with test cases and complete Doxygen-style documentation. Useful for learners studying advanced dynamic programming problems.