Add validation check for entity arg on root Query field

[tayrrible]

Description here

The entity arg should only be used on the root Query field.

This PR adds a validation check to return an error message if used on any other field.

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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. ↩

[github-actions]

@tayrrible, please consider creating a changeset entry in /.changesets/. These instructions describe the process and tooling.

[router-perf]

CI performance tests

• reload - Reload test over a long period of time at a constant rate of users
• no-tracing - Basic stress test, no tracing
• events_callback - Stress test for events with a lot of users and deduplication ENABLED in callback mode
• xlarge-request - Stress test with 10 MB request payload
• 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_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
• step - Basic stress test that steps up the number of users over time
• step-jemalloc-tuning - Clone of the basic stress test for jemalloc tuning
• 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
• step-with-prometheus - A copy of the step test with the Prometheus metrics exporter enabled
• demand-control-instrumented - A copy of the step test, but with demand control monitoring and metrics enabled
• events - Stress test for events with a lot of users and deduplication ENABLED
• xxlarge-request - Stress test with 100 MB request payload
• large-request - Stress test with a 1 MB request payload
• demand-control-uninstrumented - A copy of the step test, but with demand control monitoring enabled
• const - Basic stress test that runs with a constant number of users

[dylan-apollo]

LGTM!