Support localePrefix setting per domain

[markoleavy]

Is your feature request related to a problem? Please describe.

It would be really useful being able to configure a localePrefix preference per domain, in the domain config objects.

Describe the solution you'd like

Being able to declare a prop inside the domain object and that overrides the default behavior would fix this issue.

export default createMiddleware({
  defaultLocale: 'en-US',
  locales: ['en-US', 'fr-FR', 'en-GB', 'de-DE', 'it-IT']
  domains: [
    {
      domain: 'www.mywebsite.it',
      defaultLocale: 'it-IT',
      localePrefix: 'as-needed',
      locales: ['it-IT']
    },
    {
      domain: 'www.mywebsite.com',
      defaultLocale: 'en-US',
      localePrefix: 'always',
      locales: ['en-US', 'fr-FR', 'en-GB', 'de-DE']
    }
  ]
});

Describe alternatives you've considered

As explained in discussion #1030, at the moment is possible to use an empty string as default locale to get kind of a "always" behavior while "as-needed" is selected in main config.
This is not a documented feature, and might break in the future.