Description of problem:
copr-cli misses a command similar to koji's download-build. It would download resulted RPMs for specified copr and it could also have similar options like --arch, even if copr currently does not support more architectures.
Added command "download-build" in commit 622db5bc97e56db91d397085e4d80dc96a56299f
A bit dirty solution, since api doesn't provide direct links to rpms, so cli uses prior knowledge of file structure on server + wget recursive download.