Expose more granular wait event data to the user

[MMeent]

In PG17, there is this newfangled custom wait events system. This commit adds that feature to Neon, so that users can see what their backends may be waiting for when a PostgreSQL backend is playing the waiting game in Neon code.

Problem

We don't have very precise insights into what the backend is doing at each point in time.

Summary of changes

Add registration & naming for custom wait events where it makes sense to have them, so that we can see what the backends are waiting for.

Checklist before requesting a review

• I have performed a self-review of my code.
• If it is a core feature, I have added thorough tests.
• Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• Do not forget to reformat commit message to not include the above checklist

[github-actions]

5065 tests run: 4901 passed, 0 failed, 164 skipped (full report)

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Flaky tests (8)

Postgres 17

• test_subscriber_restart: release-x86-64

Postgres 16

• test_storage_controller_onboarding[True]: release-x86-64
• test_subscriber_restart: release-x86-64

Postgres 15

• test_lr_with_slow_safekeeper: release-arm64
• test_pageserver_metrics_removed_after_detach: release-arm64
• test_subscriber_restart: release-x86-64

Postgres 14

• test_physical_replication: release-x86-64
• test_storage_controller_onboarding[False]: release-x86-64

Code coverage* (full report)

• functions: 32.0% (7496 of 23396 functions)
• lines: 50.1% (60500 of 120865 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
22fde9d at 2024-10-01T18:17:44.613Z :recycle:}

[hlinnaka]

Ahh, I've been waiting to have these!