Preferred tarball extension to download

[Josef Schlehofer]

Hi guys,

Since commit [1], I see that OpenWrt switched to zst compression for checking out Git sources, but it looks like the conversation about enforcing package source code integrity checks [2] did not reach a conclusion (and it is problematic, though). Anyway, in OpenWrt feeds, we are downloading tarballs with .tar.xz extensions, which is preferred, as said in our CONTRIBUTING.md [3].

What should we agree upon with this?
1. Should we stick to using still .tar.xz?
2. Should we use .tar.gz or any other extensions?
However, tar.gz is bigger, but that should not be an issue at all since we are downloading these packages on hosts. However, we need to keep an eye on mirrors's storage.

This discussion is raised because package maintainers are not sure what to do, and once in a while, we switch to .tar.gz and then to .tar.xz for no reason [4]. 

[1] https://github.com/openwrt/openwrt/commit/706f0e395f5392da0a502f1039aa01ca14849ec5 
[2] https://lists.openwrt.org/pipermail/openwrt-devel/2024-April/042594.html
[3] https://github.com/openwrt/packages/blob/master/CONTRIBUTING.md#package-sources-archives-and-repositories
[4] https://github.com/openwrt/packages/pull/24299/files

Regards,
Josef