Add support for writeConcern attribute in graph API

[DiscoPYF]

In ArangoDB 3.5 there is a new property minReplicationFactor that can be set when creating graphs.

https://www.arangodb.com/docs/stable/http/gharial-management.html#create-a-graph

We should include this property in our data models.

[DiscoPYF]

minReplicationFactor has been replaced with writeConcern starting from 3.6. I will update the issue title to mention writeConcern instead. Probably not worth adding support for minReplicationFactor because it was only available in 3.5.

[tjoubert]

Solved. See #294

[DiscoPYF]

@tjoubert writeConcern option documented online when creating a graph isn't made available on PostGraphOptions. Nobody requested it, so it's probably not used much but we should reopen the issue because it is still relevant.

Options documented: https://www.arangodb.com/docs/stable/http/gharial-management.html#create-a-graph
Options available in the library: https://github.com/ArangoDB-Community/arangodb-net-standard/blob/1.1.1/arangodb-net-standard/GraphApi/Models/PostGraphOptions.cs