generic api support for Graph Computation like GraphX on Spark

[kennknowles]

Is there any plan to add support for graph computation like GraphX on Spark?

• graph representation in PCollection
• basic statistics like vertex/edge count
• base function like vertex/edge-wise mapreduce task(i.e., count the outgoing degree of a vertex)
• base function like subgraph combine/join
• ......

Imported from Jira BEAM-3096. Original Jira may contain additional context.
Reported by: rayeaster.