This PR formalizes, documents, and makes public the router outlet contract.
The set of `RouterOutlet` methods used by the `Router` has not changed
in over 4 years, since the introduction of route reuse strategies.
Creation of custom router outlets is already possible and is used by the
Ionic framework
(https://github.com/ionic-team/ionic-framework/blob/master/angular/src/directives/navigation/ion-router-outlet.ts).
There is a small "hack" that is needed to make this work, which is that
outlets must register with `ChildrenOutletContexts`, but it currently
only accepts our `RouterOutlet`.
By exposing the interface the `Router` uses to activate and deactivate
routes through outlets, we allow for developers to more easily and safely
extend the `Router` and have fine-tuned control over navigation and component
activation that fits project requirements.
PR Close#40827
a82fddf1ce