Allow Respect\Rest\Router as target.

[m42e]

Allow to define a Router as target which allows nesting of
several indipendent router. This makes routers reuseable.

See #119.

[alganet]

Almost 11 years late 🐼 This is indeed a wanted feature, but lots of things happen since then. For example, when we initially created Rest, the PSRs for HTTP didn't existed yet. The fact that now they do, and middlewares exist, potentially changes how we might approach this.

I'll investigate this use case further, and report back here. For now, I've implemented basic downstream HTTP middlewares as per PSR-15, which is not enough, but sets a direction for delegation inside the core that we might decide to reuse later.