pg_waldump: Add --save-fullpage=PATH to save full page images from WAL records

This option extracts (potentially decompressing) full-page images
included in WAL records into a given target directory.  These images are
subject to the same filtering rules as the normal display of the WAL
records, hence with --relation one can for example extract only the FPIs
issued on the relation defined.  By default, the records are printed or
their stats computed (--stats), using --quiet would only save the images
without any output generated.

This is a tool aimed mostly for very experienced users, useful for
fixing page-level corruption or just analyzing the past state of a page,
and there were no easy way to do that with the in-core tools up to now
when looking at WAL.

Each block is saved in a separate file, to ease their manipulation, with
the file respecting <lsn>.<ts>.<db>.<rel>.<blk>_<fork> with as format.

For instance, 00000000-010000C0.1663.1.6117.123_main refers to:
- WAL record LSN in hexa format (00000000-010000C0).
- Tablespace OID (1663).
- Database OID (1).
- Relfilenode (6117).
- Block number (123).
- Fork name of the file this block came from (_main).

Author: David Christensen
Reviewed-by: Sho Kato, Justin Pryzby, Bharath Rupireddy, Matthias van de
Meent
Discussion: https://postgr.es/m/CAOxo6XKjQb2bMSBRpePf3ZpzfNTwjQUc4Tafh21=jzjX6bX8CA@mail.gmail.com

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