[zip] Move class Redact to its own header file

This allows to remove duplicated code.

BUG=chromium:1303983
TEST=autoninja -C out/Default zlib_unittests && out/Default/zlib_unittests

Change-Id: I4bee3b6a901122eec20eec36f0f934b841f54588
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3515262


Reviewed-by: Alex Danilo <adanilo@chromium.org>
Commit-Queue: François Degros <fdegros@chromium.org>
Cr-Commit-Position: refs/heads/main@{#980422}
NOKEYCHECK=True
GitOrigin-RevId: 6f64fd4363bbf96ecc7de97032c831597f3bc898