New dotfile variable dotspacemacs-verify-spacelpa-archives
Default value is nil for now, users wanting to help testing this feature can
set it to t and delete the directory ~/.emacs.d/.cache/stable-elpa and restart
Emacs to download the Spacelpa archive and verify it.
Update function configuration-layer/stable-elpa-download-tarball to download
signature files as well.
Update function configuration-layer//stable-elpa-untar-archive to verify the
downloaded archive using epg library.
Example of signature files can be found here:
https://github.com/syl20bnr/spacelpa/releases/tag/v0.300
