Add support for filesystem based routing

[marcushellberg]

Is your feature request related to a problem? Please describe. A client-side routing definition has several disadvantages:

• There are multiple ways of defining routes
  • Difficult to document
  • Different across projects
  • We can't easily read it on the server to provide appropriate HTTP codes
• Users have to define access control twice: in the client-side routing file and in the Spring Security config
• It's difficult to support web crawlers with appropriate metadata on the first request
• Having a separate route definition file is boilerplate

Describe the solution you'd like I would like us to explore having a filesystem path-based routing solution similar to NextJS and other full-stack frameworks.

Additional context

At least these issues would be simplified by having the file system routing:

• Building hybrid applications
• Returning appropriate HTTP response codes for missing content
• SEO stuff like having appropriate metadata and title in the initial response
• Server-side rendering
• Users would not have to do access control to views separately in the client and Spring Security config

[Legioth]

Some notes off the top of my head based on a recent discussion:

• One particular challenge in our case is that routes are associated with view implementations in TS files whereas the metadata (e.g. access control configuration) is mostly needed by server-side Java.
  • Could run a TS parser from Java
  • Could use a neutral format (e.g. a JSON file) for the metadata at the expense of losing the direct association with the TS file implementing the view
  • Could make the TS build export metadata in a Java-friendly format (e.g. JSON) - might be problematic for development mode that is based on incremental builds
  • Could maybe rely on a 1:1 mapping between typical endpoint classes and views. In this way, the metadata could be picked up from the endpoint class - if you already have access control on an endpoint that is used by only a single view, then that view would probably have the same access rules as the endpoint.
• There's an opportunity to use a view interface similar to HasUrlParameter in Flow. In this way, the view could implement the interface and automatically get URL parameter values in a typesafe way. The same interface could also be used for navigating to a view so that parameters of the appropriate types are passed to the view.