feat: support `add_<edge>` and `remove_<edge>` in update operations

[lrstanley]

This is primarily beneficial when a given entity has thousands of edge relationships, and helps reduce the chance of an accidental call un-attaching many edges. Would also match the entgql implementation.

• Should we make it one or the other (add/remove vs all)? Just to be extra safe?
• Should we also support this on array fields? Is this even possible?