Provide an SBOM for Qt WebEngine And Qt Pdf

Currently none of the Chromium dependencies are considered when generating the SBOM for qtwebengine or qtpdf.

Not even some of the "are we using system ZLIB or Qt ZLIB" 3rd pary dependency parts of it.

As such, these qt modules are marked as "incomplete" as done in https://codereview.qt-project.org/c/qt/qtwebengine/+/577151

I tried experimenting a bit to at least cover the "qt zlib sources" part of it, but it's still WIP / POC and postponed: https://codereview.qt-project.org/c/qt/qtwebengine/+/565524

A proper SBOM for the Chromium parts should ideally come from the Chromium build system itself, aka generated by GN.

Michal from WebEngine team told me that trying to manually annotate and keep track of all the Chromium dependencies, version, licenses, etc, without relying on GN doing it for us, would likely be a full-time job, and also quite unmaintainable.

As far as I know there is no SBOM infrastructure in GN at the moment. There is only a script to output a license (and maybe copyright) list, which as far as I know QtWebEngine that feeds to qtattributionscanner to generate a docs page.

I searched online to try and see if Google plans anything, and the only relevant hit was this mailing list that suggests writing a custom script: https://groups.google.com/a/chromium.org/g/chromium-dev/c/iAhQVHP5Uag

I also filed a bug report on the Chromium bug tracker to ask about plans about SBOMs, but that got tagged with the security tag, and is thus no longer visible publicly except for me. It got assigned to someone with a google.com email, but I did not get any response so far.
https://issues.chromium.org/issues/365493101

I've also filed a GN issue: https://gn.issues.chromium.org/issues/372512097