Add support for negative edge weights to `shortest_path`

[rlratzel]

Issue https://github.com/rapidsai/cugraph/issues/4178 to add nx_cugraph.shortest_path is closed, but the current implementation does not handle negative edge weights.

This issue is for tracking progress on supporting negative edge weights. Changes are required in libcugraph and then nx-cugraph to remove the negative edge weight check which prevents execution. We should also look into updating cugraph as needed for consistency.