Deliver: @defer support in the query planner

[pcmanus]

We can implement some support for the upcoming @defer graphQL directive in the query planner by using entities keys. This will only work for a @defer that applies at the level of an entity, but this feel plenty useful: it sounds likely that a fair number of uses for @defer are going to align well with entities.

This ticket is for tracking the implementation of such entity-based @defer support in the query planner. Additional support for @defer could be later provided by passing the remaining non-entity-based defers down to subgraphs supporting it, but this should be considered out of scope for this issue. It also make sense that support for @stream be added along the same logic, but this need not be in the scope of this issue either.

[pcmanus]

Forgot to updates this earlier, but the bulk of this work was merged (and released) with #1958 so closing this issue.