api: introduce interface/function to better prepare data for graph vis

[wey-gu]

Like an interface to parse arbitrary resultset as a list of vertices and a list of edges.

With it, those who are building things to visualize graph data will be extremely happy.

cc @Nicole00 @haoxins

• [x] init impl. via #323
• [ ] support more vis graph format output flavors

[haoxins]

Absolutely~ 👍 Currently, I have to export the data from the Meta/Storage API by myself.