Feature: support of named graphs

[mvollmary]

https://docs.arangodb.com/3.3/Manual/Graphs/#named-graphs

[NingLee]

I think support of named graphs is useful.

[rashtao]

Since ArangoDB graphs API does not support overwriting (https://www.arangodb.com/docs/stable/http/gharial-edges.html#create-an-edge), it is not suitable for implementing save method in ArangoRepository, which requires insert/replace behavior.
Therefore for the moment, support to named graphs API can only be implemented in the ArangoTemplate API.