Plugin for _entities queries in Apollo federation

You might want to be able to make an `_entities` query against another federation server (perhaps one of your peers). The `_entities` field is a bit arcane for humans, but we could autogenerate the weird bits to let you make use of it; you would just write a fragment and we would put that in the right place.

No files, tests, or entry points are named. Start by examining the existing GraphQL code-generation and Apollo federation support, then define how a fragment should become an _entities query and what generated output and tests would demonstrate completion.

Written by the indexing model from the issue text.