Simplify import of signing certificates on macos executors
Apple makes it non-trivial to sign OS X applications and installers. As it stands, getting signing certificates from one development machine to another is an undertaking; getting them onto a macos executor is very difficult, especially if you're not building an XCode-based project (e.g. Electron apps), but increased security restrictions on each release of OS X makes it mandatory if you want to distribute your application. Providing a relatively easy way to import the signing certificates into a project's settings (as well as documentation on how to export the certificates properly) would be enormously helpful when building non-XCode applications and installers.