Support passing different locale to Intl APIs

[ahmedriad1]

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

Currently, all formatting logic uses the locale inferred from the context. This causes a problem when formatting requires a more specific locale code. For an example, the ar locale (Arabic) does not localize numbers into Arabic, and you have to explicitly change the locale name to ar-SA, which is not the desired url code

Describe the solution you'd like

Add a new config option to both getRequestConfig and NextIntlClientProvider called formattingLocale that is used for formatting

Describe alternatives you've considered

Manually doing rewrites in the middleware from /ar to /ar-SA internally, but that requires changing a lot of code using <Link /> and useRouter

[amannn]

This is discussed in #653. I'm aiming for built-in support for this eventually, but you can learn about workarounds that can be used currently in the issue (including a community-maintained package that wraps relevant parts from next-intl).