Validate Dynamic Route Values

[rynowak]

Summary

Background: #4221
PR: #8955

It's super common in the older versions of MVC to subclass Route and implement dynamic generation of route values (usually using a database).

Ex:

1. Write a route like product}/{**slug}
2. Store the mapping between product slugs and controller/action route values in a database
3. Implement an IRouter that does a database lookup

The key of this is that the user doesn't really write any significant routing functionality, they just want to map a parameter to a controller/action

Checklist

Example: https://github.com/aspnet/AspNetCore/pull/8955/files#diff-5a433b2664ee6b8ae7b9717bc51d3518

• Can write a DynamicRouteValueTransformer for controllers
• Can write a DynamicRouteValueTransformer for pages
• Can still use policies like [Host] or [Consumes] in conjunction with this feature