Opt-in support for locale file endpoint

[kquerna]

Description

We should have opt-in support for serving static locale files with server layer (express, fastify)? It maybe we helpful for existing apps to migrate to using the public dir with NextJS.

This option could be called serveStatic. If it is true in the config, set the endpoint to the defaultPath. We might also want allow the option to be set to a string and use that as the static endpoint. (i.e. serveStatic: '/locales'). This would be helpful if an app's default locale happens to differ from the static path.

Package(s): @gasket/plugin-intl

AC:

• Ability to configure Gasket apps with endpoint for serving static locale files
• Ability for existing apps to migrate to v6 without moving locale files