Adds infrastructure for a common IRouter-based pattern. In this pattern,
an extender subclasses Route to post-process the route values before MVC
action selection run. The new infrastructure duplicates this kind of
experience but based on endpoint routing.
The approach in this PR starts at the bottom... meaning that this is the
most-focused and least-invasive way to implement a feature like this.
Similar to fallback routing, this is a pattern built with matcher
policies and metadata rather than a built-in feature of routing.
It's valuable to point out that this approach uses IActionConstraint to
disambiguate between actions. The other way we could go would be to make
the *other* matcher policy implementations able to do this. This would
mean that whenever you have a dynamic endpoint, you will not by using
the DFA for features like HTTP methods. It also means that we need to go
re-implement a bunch of infrastructure.
This PR also adds the concept of an 'inert' endpoint - a non-Routable
endpoint that's created when fallback/dynamic is in use. This seems like
a cleaner design because we don't start *matching* RouteEndpoint
instances for URLs that don't match. This resolves#8130
Pranav K
Ryan Nowak