+str #15905 Add capability to merge and connect flow graphs

* import edges of another flow graph to the builder
* connect undefined sink with undefined source, which may originate
  from different flow graphs
* illustrate how this low level api can be used in a high level api
  (lego bricks)
Merge pull request #16016 from akka/wip-15905-interconnect-flow-graphs-patriknw

+str #15905 Add capability to merge and connect flow graphs