Support to connect to GraphDB (e.g AllegroGraph)

[dasrajkumar52]

Is your feature request related to a problem? Please describe.
Our backend database can be a graph database. We want this library to support to connect to graph DB for data fetch. One of the example of

Describe the solution you'd like
It can connect to graph db to fetch data and we can manage the schema to fetch the data accordingly.

[paveltiunov]

@hackrepo Hey Rajkumar! Thanks for posting this! There's a guide if you want to contribute it: https://github.com/cube-js/cube.js/blob/master/CONTRIBUTING.md#implementing-driver