Ensure subgraph requests have access to their corresponding ExecutableDocuments

[tninesling]

When we apply demand control measurements, we want to measure the cost of an operation at several points. We score the overall operation, the query plan, and each subgraph operation. In order to score these operations, we need to use an ExecutableDocument, which is already provided for the top-level operation but not for the subgraph requests.

In order to achieve this, we could store the ExecutableDocument on each subgraph request. However, this defeats the purpose of having the fetch nodes lazily initialize the documents (and breaks a bunch of query planner tests because they are lacking appropriate subgraph schemas in their setup). To avoid having to define subgraph schemas for all these tests and to preserve the lazy initialization setup, I instead embed a reference to the fetch node that is creating the subgraph request, which can trigger the lazy init when it is being executed.

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Complete the checklist (and note appropriate exceptions) before the PR is marked ready-for-review.

• [ ] Changes are compatible[^1]
• [ ] Documentation[^2] completed
• [ ] Performance impact assessed and acceptable
• Tests added and passing[^3]
  • [X] Unit Tests
  • [ ] Integration Tests
  • [ ] Manual Tests

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CI performance tests

• [x] step - Basic stress test that steps up the number of users over time
• [ ] events_big_cap_high_rate_callback - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity using callback mode
• [ ] large-request - Stress test with a 1 MB request payload
• [ ] events - Stress test for events with a lot of users and deduplication ENABLED
• [ ] xxlarge-request - Stress test with 100 MB request payload
• [ ] events_without_dedup - Stress test for events with a lot of users and deduplication DISABLED
• [ ] xlarge-request - Stress test with 10 MB request payload
• [ ] step-jemalloc-tuning - Clone of the basic stress test for jemalloc tuning
• [ ] events_callback - Stress test for events with a lot of users and deduplication ENABLED in callback mode
• [ ] no-graphos - Basic stress test, no GraphOS.
• [ ] reload - Reload test over a long period of time at a constant rate of users
• [ ] events_big_cap_high_rate - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity
• [ ] events_without_dedup_callback - Stress test for events with a lot of users and deduplication DISABLED using callback mode
• [x] const - Basic stress test that runs with a constant number of users

[tninesling]

I'm going to close this for now so we can focus on metrics for the execution service in this plugin. We can revisit this later when we want to actually implement the subgraph service