fixup(#7168): neon_local: use pageserver defaults for known but unspecified config overrides

[skyzh]

Problem

e2e tests cannot run on macOS unless the file engine env var is supplied.

./scripts/pytest test_runner/regress/test_neon_superuser.py -s

will fail with tokio-epoll-uring not supported.

This is because we persist the file engine config by default. In this pull request, we only persist when someone specifies it, so that it can use the default platform-variant config in the page server.

Summary of changes

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]

2700 tests run: 2570 passed, 0 failed, 130 skipped (full report)

---

Code coverage* (full report)

• functions: 28.3% (7128 of 25166 functions)
• lines: 46.8% (43694 of 93287 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
0dc7fb3 at 2024-03-18T21:11:48.870Z :recycle:}

[problame]

Good catch!

Please apply the same changes to get_vectored_impl, and adjust this PR's title & description to sth like

fixup(#7120): neon_local: use pageserver defaults for known but unspecified config overrides