Runtime hook for breadth execution patterns

[gmac]

This adds a hook into the execution runtime that allows an evaluated selection to exit with its resolved inner value. This allows a breadth-first execution process to run a single generation of resolvers at a time across a set of objects, ex:

This general breadth capability relies on two runtime methods:

• evaluate_selection(result_key, ast_nodes, selections_result)
• exit_with_inner_result?(inner_result, result_name, selection_result)

The expected capabilities are covered by a simple example of a breadth-based runtime in tests. This is still an experimental workflow, so it seems worthwhile to keep the new code footprint minimal for the time being (just exit_with_inner_result?) and in a state that can be backed out easily.

In the future, we'd ideally formalize a structure like this SimpleBreadthRuntime as a supported library feature. The actual breadth prototype I have in the works uses this same basic design while caching breadth objects across fields and consolidating the lazy hooks across scopes. It appears capable of passing some existing test suites while running slightly faster than native depth execution.

[gmac]

In the actual prototype, I’m running dataloader inline with the field but this lazy step runs out-of-band after a bunch of fields have been queued. Sounds like with #5422, I’d just run dataloader once in that out-of-band position?

[gmac]

In the actual prototype, breadth results are keyed with a unique representation of a field path.