Support implicit graph specifications in GCS (taking source + `GetSuccessor()`)

[RussTedrake]

It is possible to solve GCS problems where the original graph is infinite, or too large to fit in memory. A handful of recent papers have studied incremental search algorithms for GCS (c.f. GCS*). In order to support the implementation of these algorithms, we must first capture the incremental GCS specification in a class, named something likeImplicitGraphOfConvexSets (though I would favor a name that started with GCS instead of ended with it, to improve discovery).

cc @rhjiang, @iamsavva, @bernhardpg who are also very involved/interested in this.