Merge pull request #39526 (improve dhparams)

This introduces an option that allows us to turn off stateful generation
of Diffie-Hellman parameters, which in some way is still "stateful" as
the generated DH params file is non-deterministic.

However what we can avoid with this is to have an increased surface for
failures during system startup, because generation of the parameters is
done during build-time.

Aside from adding a NixOS VM test it also restructures the type of the
security.dhparams.params option, so that it's a submodule.

A new defaultBitSize option is also there to allow users to set a
system-wide default.

I added a release notes entry that described what has changed and also
included a few notes for module developers using this module, as the
first usage already popped up in NixOS/nixpkgs#39507.

Thanks to @Ekleog and @abbradar for reviewing.