Support opaque mode for log_newpages

[knizhnik]

Problem

DuckDB pages do not contains page header.
See https://neondb.slack.com/archives/C0799BZALLR/p1722148143356959?thread_ts=1722030414.537519&cid=C0799BZALLR

The problem: we need to wallog huge (256kb) DuckDB pages.
It is natural to use Postgres log_newpages() function which insert FPI record.
The problem is that this function assumes that pages has Postgres PageHeader.
It is already has std_page parameter (where pd_lower,pg_upper specify hole inside page).
But even non-standard page is assumed to have PageHeader and Postgres is setting LSN for it.

So there are two other alternatives:

1. Split DuckDB 256kb page in 33 Postgres 8kb pages excluding header. Last page will be almost empty. And overall this approach looks ugly.
2. Add special resource manager for DuckDB. It requires much more coding and cause writing a lot of boilerplate and duplication of a lot of code from xlog.c

First looks ugly, second requires no changes in Postgres, but a lot of copy&paste of Postgres and Neon code.

Summary of changes

Add REGBUF_OPAQUE and pass flags to wal_newpages instead of std_page parameter.

Correspondent PR for Postgres:
neondatabase/postgres#456

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]

2108 tests run: 2039 passed, 0 failed, 69 skipped (full report)

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Code coverage* (full report)

• functions: 32.9% (7156 of 21726 functions)
• lines: 50.6% (57779 of 114232 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
bb86bd2 at 2024-08-05T12:50:57.291Z :recycle:}

[ololobus]

Please

• add a link to the corresponding Postgres PR to this PR description
• reference the issue in both PRs -- https://github.com/neondatabase/cloud/issues/15361

Cannot review the code part of the PR, so added Heikki as well