[EventDispatcher] Add an interface for the ContainerAwareEventDispatcher

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        | 

Currently if a class depends on the ContainerAwareEventDispatcher it's not possible to replace it by the TraceableEventDispatcher without rewriting the class.
I think they remain protected due to BC. Personally I'm against protected because it's pretty close to making it public, thus killing encapsulation. As far as I know, `@api` is for methods, that's what you talk to, even when extending.