Next.js proxy library

This proxy replaces traditional XP rendering, and rather acts as a proxy that communicates with remote rendering servers based on Next.js. Beyond acting as a basic proxy, it manipulates hardcoded Next.js assets with the `/_next` prefix to work with Content Studio. The proxy only solves a part of making links work well in Content Studio. The Next.js implementation must also make use headers passed by the proxy to generate contectually valid links.