JSONSelection in URIs and Header values

[dylan-apollo]

This PR implements full expressions in URIs and header values (e.g., arrow methods, not just variables) via a few pieces:

1. The new apollo-federation/src/sources/connect/string_template.rs contains shared logic for both URIs and headers, so they can be more consistent behavior (both parsing and interpolation). This comes with support for full JSONSelection rather than just the variable references
2. As a result of 1, headers now work much like URIs in that arrays and objects won't be automatically serialized to JSON, instead it is an error for an expression to result in one of those. This is a breaking change. Users should work around this with ->jsonStringify from ->jsonStringify method #6519
3. Much of the former variable reference code has been removed in favor of this new parsing approach, which leans on the JSONSelection parser instead
4. To validate these more complex expressions, the new shape library is being used. As a result this PR relies on Implement ApplyToInternal::compute_output_shape and JSONSelection::shape returning shape::Shape #6458

Pay extra attention to the test cases in apollo-federation/src/sources/connect/validation/expression.rs to look for any gaps. In particular, any named types ($args, $this, $config, etc.) can have unexpected effects on the shape processing as things need to be checked/simplified before the types are known.

There are some expected gaps in the validations where we need to build more infrastructure for the type checker. For example, ->map is completely ignored for now.

[svc-apollo-docs]

✅ Docs preview has no changes

The preview was not built because there were no changes.

Build ID: 4f9769aff1fc03c607ee1919

[router-perf]

CI performance tests

• connectors-const - Connectors stress test that runs with a constant number of users
• const - Basic stress test that runs with a constant number of users
• demand-control-instrumented - A copy of the step test, but with demand control monitoring and metrics enabled
• demand-control-uninstrumented - A copy of the step test, but with demand control monitoring enabled
• enhanced-signature - Enhanced signature enabled
• events - Stress test for events with a lot of users and deduplication ENABLED
• 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_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
• events_callback - Stress test for events with a lot of users and deduplication ENABLED in callback mode
• events_without_dedup - Stress test for events with a lot of users and deduplication DISABLED
• events_without_dedup_callback - Stress test for events with a lot of users and deduplication DISABLED using callback mode
• extended-reference-mode - Extended reference mode enabled
• large-request - Stress test with a 1 MB request payload
• no-tracing - Basic stress test, no tracing
• reload - Reload test over a long period of time at a constant rate of users
• step-jemalloc-tuning - Clone of the basic stress test for jemalloc tuning
• step-local-metrics - Field stats that are generated from the router rather than FTV1
• step-with-prometheus - A copy of the step test with the Prometheus metrics exporter enabled
• step - Basic stress test that steps up the number of users over time
• xlarge-request - Stress test with 10 MB request payload
• xxlarge-request - Stress test with 100 MB request payload

[pubmodmatt]

Nitpick: maybe an additional test for properties nested at a greater depth, like:

#[case::multi_level_input("$args.multiLevel.inner.nested")]

(I did try this and it worked)