Support CF Route Services

I started writing this up as a post on the spring-cloud Gitter, but thought it might make more sense to post somewhere a little more permanent.
I recently wrote a blog post about using spring cloud gateway to create a route service. First off/selfishly, I was wondering if anyone had suggestions on alternate approaches I could/should have taken to implement the route service.

Secondly, I wanted to bring up a couple ergonomic issues and see if they would make sense to create separate issues/contributions for:

• Should it be possible to create a route without a URI? This might make sense if the final destination is determined within a filter
• Does a single arg header filter make sense? This would just check for the existence of a header... instead of header("X-My-Header", ".*"), it would be header("X-My-Header")
• in GatewayFilterSpec:
  • Should add and filter both exist? It feels a little weird that they both exist w/ the same functionality
  • If add and filter are meant to be aliases, should it be possible to specify an order using add?
  • Should there be a varargs version of addAll? Adding multiple filters is currently a little awkward
  • Would it make sense to automatically get the order for filters which implement Ordered?

Start by reviewing GatewayFilterSpec and the route-creation behavior described in the issue, then compare the proposed route-service use case with the linked blog post. The scope is not yet settled: completion would require a decision on URI-less routes and the filter API questions before implementation can be defined.