Improve PluginTestHarness for multi call (backport #6623)

[BrynCooke]

The PluginTestHarness previously created services on every call.
However this is no good for testing plugins in the new world where services are reused.

The harness has been modified to return service handles that allow calls to be made against the service:

let test_harness: PluginTestHarness<MyTestPlugin> =
            PluginTestHarness::builder().build().await;

        let mut service = test_harness.router_service(|_req| async {
            Ok(router::Response::fake_builder()
                .data(serde_json::json!({"data": {"field": "value"}}))
                .header("x-custom-header", "test-value")
                .build()
                .unwrap())
        });

        for _ in 0..2 {
            let response = service.call_default().await.unwrap();
            assert_eq!(
                response.response.headers().get("x-custom-header"),
                Some(&HeaderValue::from_static("test-value"))
            );
        }

---

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

---

This is an automatic backport of pull request #6623 done by [Mergify](https://mergify.com).

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. ↩

[mergify]

Cherry-pick of 5915d74 has failed:

On branch mergify/bp/1.x/pr-6623
Your branch is up to date with 'origin/1.x'.

You are currently cherry-picking commit 5915d746.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   apollo-router/src/configuration/snapshots/apollo_router__configuration__tests__schema_generation.snap
	modified:   apollo-router/src/plugins/demand_control/mod.rs
	modified:   apollo-router/src/plugins/telemetry/config_new/graphql/mod.rs
	modified:   apollo-router/src/plugins/telemetry/logging/mod.rs
	modified:   apollo-router/src/plugins/test.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   apollo-router/src/plugins/limits/mod.rs
	both modified:   apollo-router/src/plugins/telemetry/config_new/events.rs

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[svc-apollo-docs]

✅ Docs preview has no changes

The preview was not built because there were no changes.

Build ID: f7775963d5a3a217612e83d8

[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