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

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