Add Graph Provider to support ALEC

We have a feature called ALEC - it maintains a graph in memory: https://github.com/OpenNMS/alec/blob/develop/engine/cluster/src/main/java/org/opennms/alec/engine/cluster/GraphManager.java#L71

We want that graph to be exposed via the OIA to the new Graph Service API, so that it is accessible in the Topology UI

So, a on system with some existing nodes, load ALEC and the ALEC topology should appear in the UI (in the background this is OIA -> GraphService -> Topology App)