Consolidate & expand connectors headers validations

[dylan-apollo]

Before this PR, there were four separate validations for headers:

1. At composition, header names and values are parsed to check for validity. This also checks whether to and from are both set or just one of them, and disallows duplicate headers.
2. When the router loads a schema, it has a separate, less thorough check for argument validity on @source
3. When the router loads a schema, it also checks @connect headers in a similar code path, but one that didn't allow single header objects yet.
4. At runtime, certain header names are disallowed in some contexts

This PR unifies the base definition of what's considered a valid headers argument, catching disallowed header names at composition time and at router schema load (so there's no need to validate again at runtime).

Improves the locations of a number of validations, pointing at the specific header that errored rather than the whole headers block.

[svc-apollo-docs]

✅ Docs Preview Ready

No new or changed pages found.

[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]

Looks good overall! Just one change requested on a new header error type.