Allow targeting by operating system

[kkostov]

It should be possible to provide links specific to a given operating system. For example:

{
  "@context": "http://schema.org",
  "@type": "CreativeWorkSeries",
  "hasPart": [
    {
      "@type": "SoftwareApplication",
      "provider": {
        "@type": "Organization",
        "name": "Apple"
      },
      "operatingSystem": "macOS", <------ ADDED
      "potentialAction": {
        "@type": "ViewAction",
        "target": {
          "@type": "EntryPoint",
          "urlTemplate": "https://apps.apple.com/app/id12345"
        }
      }
    }
  ]
}