[FEA]: Add `from_edgelist` and `G.add_edges_from`

These are commonly used in workflows and downstream libraries. We may be able to reuse code in `from_networkx` to do most of the heavy lifting.

To make this most usable, we should be able to do:
```python
>>> G = nxcg.Graph()
>>> G.add_edges_from(edges)  # Still on GPU; not on CPU
```

Related functions (should be separate issues if they need tracked): `to_edgelist` and `from_dict_of_dicts`.