Allow decorating route controllers

I'm torn on the services/routing.yml issue. I can see the sense in using services, because the concept of decoration already exists there and some of the implementation comes for free.
But for DX, I think on balance I prefer the DX of 'decorated routes are defined where you expect routes to be defined' rather than 'decoration of routes happens where you expect decoration of things to be defined', because conceptually the important thing is that this is do with routing, rather than with decoration.

But I think that 📌 Use PHP attributes for route discovery Needs review will make this discussion obsolete -- once that issue is in, we can define route decoration using attributes.

I think it's better status to focus efforts on 📌 Use PHP attributes for route discovery Needs review