Implement endpoint to read in- or outbound edges

[DiscoPYF]

It looks like one of the HTTP API endpoint related to graphs is not implemented: GET /_api/edges/{collection-id}.

https://www.arangodb.com/docs/stable/http/edge.html#read-in--or-outbound-edges

I propose to create a new GetEdges method under IGraphApiClient/GraphApiClient. While all methods in GraphApiClient are currently for the _api/gharial route, we can consider this new endpoint to be part of the overall "Graph API", which is what GraphApiClient is for.

[DiscoPYF]

We could also create a new IEdgesApiClient if we want to follow the same separation as the API routes. But it may be overkill for just one endpoint.