Support Dgraph Geo type

[EnricoMi]

Dgraph encode geo coordinates via GeoJSON standard. Currently, the connector represents this as a JSON string, but it should be a proper struct with type and coordinates fields. Depending on the type, the coordinates field has a different type. Example Scala code: https://gist.github.com/iamhatesz/4f4b83fd32c52f002766f9a70c2f6a71.

Maybe JSON is fine, Spark supports parsing that string into a struct, maybe we can hook into that and provide the parsed JSON directly.