feat(pageserver): add direct io pageserver config

[yliang412]

Part of #8130, RFC: Direct IO For Pageserver

Description

Add pageserver config for evaluating/enabling direct I/O.

• Disabled: current default, uses buffered io as is.
• Evaluate: still uses buffered io, but could do alignment checking and perf simulation (pad latency by direct io RW to a fake file).
• Enabled: uses direct io, behavior on alignment error is configurable.

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]

2112 tests run: 2043 passed, 0 failed, 69 skipped (full report)

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Code coverage* (full report)

• functions: 32.5% (7157 of 22003 functions)
• lines: 50.5% (57850 of 114500 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
7b0689c at 2024-08-07T19:54:02.903Z :recycle:}

[skyzh]

you may also change the default neon_local config (in some toml string in the codebase) to enable it by default for neon local users.

[yliang412]

you may also change the default neon_local config (in some toml string in the codebase) to enable it by default for neon local users.

I will change the config once I'm certain with my implementation in the read path.