feat: introduce graph InEdges & OutEdges methods

[atzoum]

Up to now, if one was interested in getting all edges of a source or target, there used to be two options:

• Use the Edges method and filter the result based on source/target.
• Use the AdjacencyMap/PredecessorMap method, select the appropriate key and transform the map into a slice

Both of these options have proved to be quite inefficient for very big graphs, e.g. with millions of edges.

Therefore we are introducing two new methods which take advantage of the store's internal structures:

1. InEdges: returns a slice of all edges in the graph with a specific target vertex
2. OutEdges: returns a slice of all edges in the graph with a specific source vertex.

[atzoum]

@dominikbraun we are facing some significant performance issues with the graph module and this change can unblock us 🙏

[dominikbraun]

Hey, so this seems to change both the Graph and Store interfaces and we'll need a new major version since it's a breaking change. I'll have a look...