smith: generate a list of subgraphs

[lrlna]

Add functionality to apollo-smith to generate an arbitrary number of subgraphs. This shouldn't compose them into a supergraph.

I think it makes a bit more sense to have this api to return a list, since subgraphs have to reference each other a little bit in order to make an "interesting" for fuzzing supergraph. it might be easier if that's done in a single process. happy to hear other thoughts though!