support Dgraph data source

[Nicole00]

No description provided.

[Nicole00]

Use Spark Connector to read dgraph data and write to nebula. for example: https://github.com/vesoft-inc/nebula-spark-connector/blob/master/example/src/main/scala/com/vesoft/nebula/examples/connector/NebulaSparkWriterExample.scala

1. In user's project, add the dependency in pom:

<dependency>
  <groupId>uk.co.gresearch.spark</groupId>
  <artifactId>spark-dgraph-connector_2.12</artifactId>
  <version>0.4.2-2.4</version>
</dependency>

2. read dgraph data

import uk.co.gresearch.spark.dgraph.connector._
val edges: DataFrame = spark.read.dgraphEdges("localhost:9080")
val nodes: DataFrame = spark.read.dgraphNodes("localhost:9080")

3. write to nebula

df.write.nebula(config, nebulaWriteVertexConfig).writeVertices()

[Nicole00]

do not integrate dgraph to Exchange. Reasons:

1. The requirement is not general. Dgraph stores RDF, and use gql, which is used in knowledge graph, and now we don't have many requirements for it.

2. Besides, there's other resolutions (showed above) for users to use.