feat: add Huffman coding algorithm to greedy_approach

[DiogoRibeiro7]

Description of Change

Add a Huffman coding compression algorithm implementation to the greedy_approach/ directory. Huffman coding is a classic greedy algorithm that assigns variable-length binary codes to characters based on their frequencies, producing an optimal prefix-free code.

The implementation includes:

• A min-heap (priority queue) for building the Huffman tree
• Tree construction by repeatedly merging the two lowest-frequency nodes
• Recursive code generation via tree traversal (left = '0', right = '1')
• Special case handling for single unique character inputs
• Interactive mode that displays character codes and compression statistics
• 5 self-test cases covering basic encoding, prefix-free property, single character, equal frequency, and optimality

Also adds the missing CMakeLists.txt for the greedy_approach/ directory and registers it in the root CMakeLists.txt.

References

N/A - new algorithm addition, no prior issue or PR.

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• 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: Adds greedy_approach/huffman.c with full Doxygen documentation, min-heap based tree construction, and 5 assert-based tests. Also creates greedy_approach/CMakeLists.txt which was previously missing.