Download `gpg` on Mac if not available

[abitrolly]

While it is possible to install gpg with brew #14 #27, it kind of defeats the purpose of using less package managers. Missing gpg creates problems only for Mac users, because most Linux systems have it by default - https://gitlab.com/gitlab-org/gitlab-docs/-/issues/1195

asdf-flutter uses this approach for jq - https://github.com/oae/asdf-flutter/blob/master/bin/jq-downloader

EDIT: Press 👍 if you want yarn installer to manage its need for gpg transparently and automatically. I made the first thumb for you.

[abitrolly]

Or disable signature validation if gpg is unavailable. If somebody was able to get forged binary to download server with https:// certificate, that somebody would also alter the validation key which is downloaded from there.

[pesterhazy]

I want to second this request. No other asdf plugin that I'm aware of has an external requirement that's not typically installed on macOS.

We had to revert a change to get yarn via asdf because of this small wart.