Add Neo4J APOC import/export JSON format

[nichtich]

The Neo4J APOC import/export JSON format is similar to PG-NJSON. It differs in the following aspects:

• edge labels are strings instead of array of strings each edge has exactely one label
• property values can also be scalar instead of array of scalars and empty arrays are allowed as well
• property values can also be extended types serialized as JSON object
• key property is omitted in nodes and edges without properties
• edge keys are named start and end instead of from and to and they hold a JSON object with node id and labels instead of the plain identifier