MacOS signing

[maxkatz6]

Steps:

• Minimal sign scripts, figuring out how to use Cake.
• Update AppVeyor to run mac build on MacOS machine. This will automatically solve "chmod +x" problem too.
• Use p12 and password from the secrets/env.
• Validate that signing actually works
• Try writing notarizing script (it might timeout on CI with JIT entitlements)

Optional, to be done in other PRs:

• Use NativeAOT in the release builds, remove JIT entitlements, try notarizing again
• Create DMG file

Closes #686

[maxkatz6]

Ah, okay, it runs now, after I double checked syntax with https://ci.appveyor.com/tools/validate-yaml
Weirdly it didn't tell any errors on commit status.