Translate non-string values in the `<Dropdown.Native>`

[caillou]

The <Dropdown.Native> abstraction is leaky. Currently, only string values work.

Ideally, users of the style-guide should not care about this, e.g.: orbiting/republik-admin-frontend#59 (comment)

if (typeof value !== 'string') {
  // map a random ID to the value.
}