feat(pagebench): add aux file bench

[skyzh]

Problem

part of #7462

Summary of changes

This pull request adds two APIs to the pageserver management API: list_aux_files and ingest_aux_files. The aux file pagebench is intended to be used on an empty timeline because the data do not go through the safekeeper. LSNs are advanced by 8 for each ingestion, to avoid invariant checks inside the pageserver.

For now, I only care about space amplification / read amplification, so the bench is designed in a very simple way: ingest 10000 files, and I will manually dump the layer map to analyze.

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]

3078 tests run: 2951 passed, 0 failed, 127 skipped (full report)

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Flaky tests (3)

Postgres 15

• test_compute_pageserver_connection_stress: release

Postgres 14

• test_synthetic_size_while_deleting: debug
• test_timeline_size_quota_on_startup: release

Code coverage* (full report)

• functions: 31.3% (6382 of 20401 functions)
• lines: 47.7% (48631 of 101897 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
f106bdd at 2024-05-17T20:11:49.253Z :recycle:}

[problame]

The new pagebench isn't measuring anything, and the workload it applies isn't parametrizable via CLI. It seems quite incomplete to me.

I'm ok with merging it if you intend to extend it later, would be nice to link the follow-up issue for that.

Approval is subject to changing the POST API to testing_api_handler, I don't think it's safe to use if there's a concurrent other DatadirModifications.