C++ implementation of Prim's Minimum Spanning Tree Algorithm

[prex03]

This PR adds:

1. C++ implementation of Prim's MST algorithm using a priority queue on adjacency list graphs.
2. std::variant-based edge values in GraphEdge, replacing PyObject* for better type safety and performance.
3. Includes tests to verify MST correctness and edge value handling.

[czgdp1807]

Conflicts need to be resolved here. @prex03 Please complete it.

[prex03]

Conflicts need to be resolved here. @prex03 Please complete it.

Done.

[czgdp1807]

Points to be considered,

1. Try abseil containers, instead of std::unordered_map, std::unordered_set.
2. Use, integer IDs in C++ backend for algorithms instead of std::string names.
3. Find a better representation for undirected graphs (right now in AdjacencyListGraph, a -- b is represented via {a: [b], b: [a]}