arccore.graph support developer-defined serializable context object attached to DirectedGraph

[ChrisRus]

DirectedGraph provides support for vertex and edge "properties" - arbitrary developer-defined data attached to vertices and edges in a DirectedGraph container instance. This is useful. But, oftentimes you're building a graph based on some data structure produced by another source and you're effectively indexing it. i.e. you're not transforming the input into an equivalent graph representation but rather building a graph to keep track of some feature(s) of the input data.
In these cases, we want to use internal properties to hold metadata references back to source data (e.g. data structure paths, keys, etc). And, we want to maintain the input as it is so that the information stored in the derived graph can be used to dereference back to the data.
What this feature allows is for the external data to be associated with the derived graph such that serializing the derived graph also serializes the "context" the graph is associated with.