[RFC] Full Next.Js support and UI upgrades

Full Next.JS support

Currently the frontend is hidden away in .keystone/admin folder and generates multiple pages for each list routes. This is not very continent and makes customization very difficult. To overcome this I have proposed use of full Next.Js integration.

Some initial poc done in #9181 .

Goals

• Leverage full Next.Js integration instead of isolated instance
• Upgrade Next.Js to use App Router with server actions
• Code split custom field views instead of bundling them together
• Frontend to have less dependency on GraphQL for Administration, this also opens up additional features
• upgrade @keystone-ui to be ssr friendly
• Decouple Admin-UI and frontend logic to allow use of other frameworks like Remix
• Allow selection of relationship fields in UI
• Nested filters in UI
• Add support for UI Hooks/Extensions
• ....TBD (waiting for community feedback)

Using App router will allow theming and customization easily with the Layouts.

Kudos to @junaid33 for initial Next.Js 14 work done in https://github.com/openshiporg/openfront