Accessing `pgClient` in hooks for `explain` data
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### Summary

We're using the embeddable Apollo Explorer in place of GraphiQL (for a number of reasons that are off topic) and we'd like to add the `explain` data onto result data.

### Additional context

We already know that we can use the `postgraphile:http:end` hook to mutate result data. However, we're hitting a wall trying to find the right hook or right method to access `pgClient._explainResults`. We're starting postgraphile with `allowExplain: true` fwiw.

I've tried to trace through contexts to figure out where postgraphile might be exposing that, but I'm coming up empty. (It's a bit dizzying trying to follow what is sent where and how contexts are constructed, without being intimately familiar with it).

Any pointers to solve this one?
