feat(extensions): add install extensions from registry

[kjuulh]

This pr includes everything up to actually downloading the extension from the upstream registry

Such as pulling an example registry entry

{
  "name": "SuperExtender",
  "description": "Enhances your experience with superpowers.",
  "version": "1.0.3",
  "downloadUrls": [
    {
      "architecture": "amd64",
      "os": "linux",
      "url": "https://example.com/linux/amd64/superextender.tar.gz",
      "checksum": "efgh5678"
    },
    {
      "architecture": "x86",
      "os": "windows",
      "url": "https://example.com/windows/x86/superextender.zip",
      "checksum": "ijkl91011"
    },
    {
      "architecture": "arm64",
      "os": "darwin",
      "url": "https://example.com/windows/x86/superextender.zip",
      "checksum": "ijkl91011"
    }
  ]
}

Which will match on the current os using $GOOS and $GOARCH set by golang itself. We may however have to depend on some other variables if go is not installed by I believe it should work anyway"

Builds on: #222