fix(pageserver): pagebench doesn't work with released artifacts

[problame]

The canonical release artifact of neon.git is the Docker image with all
the binaries in them:

docker pull neondatabase/neon:release-4854
docker create --name extract neondatabase/neon:release-4854
docker cp extract:/usr/local/bin/pageserver ./pageserver.release-4854
chmod +x pageserver.release-4854
cp -a pageserver.release-4854 ./target/release/pageserver

Before this PR, these artifacts didn't expose the keyspace API,
thereby preventing pagebench get-page-latest-lsn from working.

Having working pagebench is useful, e.g., for experiments in staging.
So, expose the API, but don't document it, as it's not part of the
interface with control plane.

[github-actions]

2436 tests run: 2317 passed, 0 failed, 119 skipped (full report)

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Flaky tests (4)

Postgres 16

• test_create_snapshot: debug
• test_timeline_deletion_with_files_stuck_in_upload_queue: debug

Postgres 15

• test_create_snapshot: release
• test_statvfs_pressure_usage: release

Code coverage (full report)

• functions: 56.0% (12850 of 22964 functions)
• lines: 82.6% (69346 of 83998 lines)

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_{The comment gets automatically updated with the latest test results
050fb31 at 2024-02-14T16:34:49.743Z :recycle:}