optimal binary search tree added in dynamic_programming

[butterpaneermasala]

Description

This PR adds an implementation of the Optimal Binary Search Tree (OBST) algorithm in Rust under the dynamic_programming module.

The algorithm uses dynamic programming with prefix sum optimization to reduce time complexity to O(n³). It computes the minimum expected search cost for a binary search tree, given a set of frequencies that represent how often each key is accessed.

For more details about the algorithm: Optimal Binary Search Tree – GFG

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

Checklist

• I ran the below commands using the latest version of rust nightly.
• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• I ran cargo fmt just before my last commit.
• I ran cargo test just before my last commit and all tests passed.
• I added my algorithm to the corresponding mod.rs file within its own folder, and in any parent folder(s).
• I added my algorithm to DIRECTORY.md with the correct link.
• I checked CONTRIBUTING.md and my code follows its guidelines.

Please make sure that if there is a test that takes too long to run ( > 300ms), you #[ignore] that or
try to optimize your code or make the test easier to run. We have this rule because we have hundreds of
tests to run; If each one of them took 300ms, we would have to wait for a long time.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.48%. Comparing base (a3b116d) to head (5fd66b7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #890      +/-   ##
==========================================
- Coverage   95.51%   95.48%   -0.04%     
==========================================
  Files         318      319       +1     
  Lines       22975    23014      +39     
==========================================
+ Hits        21945    21974      +29     
- Misses       1030     1040      +10     

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