Add support for using comps groups for dnf download

[Conan-Kudo]

Currently, dnf download does not support comps groups as arguments like other commands do. Extending it to support comps groups would make it much easier to create filtered mirrors by downloading subsets of the repository content.

Please add support for this.

This would also mean that there should be a dnf group download command too.

While there are some workarounds for this by using dnf install @'<group-name>' --downloadonly --installroot <empty_dir> --destdir <custom_dir>, the problem with it is that you have to be root to run this, which is not particularly great if you're trying to mirror as a non-root user.