Support multiple installer/bundle types per platform

[FabianLars]

Now with the introduction of .deb updater support our current json schema really shows its limits. It would also be a valuable addition for Windows though because support msi and exe sometimes makes sense (or at least so i was told).

About the Static JSON response changes required:
Duplicate object keys are seemingly not well supported (understandibly since the specs highly discourage it) so i guess that's sadly out.

I feel like overloading the current object keys like x86_64-windows-nsis is a bit ugly but honestly not the worst.

Alternatively we could provide basically schema version 2 and use an array instead of an object, example:

{
  // Maybe rename platforms to better reflect what the array actually contains
  "platforms": [
    {
      "os": "windows",
      "arch": "x86_64",
      "bundleType": "nsis",
    },
    {
      "os": "windows",
      "arch": "x86_64",
      "bundleType": "wix",
    }
  ]
}