feature: use minisign to sign binaries

[holiman]

From Thomas Ptacek (https://latacora.singles/2019/07/16/the-pgp-problem.html ) :

Signing Packages

Use Signify/Minisign. Ted Unangst will tell you all about it. It’s what OpenBSD uses to sign packages. It’s extremely simple and uses modern signing. Minisign, from Frank Denis, the libsodium guy, brings the same design to Windows and macOS; it has bindings for Go, Rust, Python, Javascript, and .NET; it’s even compatible with Signify.

About minisign: https://jedisct1.github.io/minisign/

Minisign is compatible with signify used by OpenBSD

The todo's would be:

• Make some dummy key and make available to builders
• Make the builders generate the signature file, based off the dummy key
• Swap to a real key, publish pub sig on download page