Add ability for router to deal with query plans with contextual rewrites

[clenfest]

Two main things that we're doing in this PR.

1. We've added a variable to FetchNode called context_rewrites. This is a vector of DataRewrite::KeyRenamer that are specifically taking data from their path (which will be relative and can traverse up the data path) and writes the data into an argument that is passed to the selection set.
2. There are two cases. In the most straightforward, the data that is passed to the selection set is the same for every entity. This case is pretty easy and doesn't require any special handling. In the second case, the value of the variable may be different per entity. If that is true, we need to use aliasing and duplication in our query in order to send it to subgraphs. Once Finding the right Batching Mechanism graphql/composite-schemas-spec#25 is decided and has subgraph support, this query cloning will be able to go away.

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Checklist

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

• Changes are compatible¹
• Documentation² completed
• Performance impact assessed and acceptable
• Tests added and passing³
  • Unit Tests
  • Integration Tests
  • Manual Tests

Exceptions

Note any exceptions here

Notes

Footnotes

1. It may be appropriate to bring upcoming changes to the attention of other (impacted) groups. Please endeavour to do this before seeking PR approval. The mechanism for doing this will vary considerably, so use your judgement as to how and when to do this. ↩

2. Configuration is an important part of many changes. Where applicable please try to document configuration examples. ↩

3. Tick whichever testing boxes are applicable. If you are adding Manual Tests, please document the manual testing (extensively) in the Exceptions. ↩

[router-perf]

CI performance tests

• 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
• const - Basic stress test that runs with a constant number of users

[garypen]

Great work on this and thanks for adding the extra tests and cleanup.

I have one comment about alpha federation release, which I don't think we've done before in the router, but apart from that I'm happy.

[garypen]

Are we happy releasing a router which is using a federation alpha release? Is the intent to update this before we release the next version of the router?

[clenfest]

Yes. This is just to let us bake in a bit more testing before we officially release 2.8.0, but if all goes well we can just promote it. We'll certainly release and bump the pin before we cut a 1.48.0 rc