Replaces the existing, very specialized Notification with something more
general.
Currently, the existing page_navigate and page_navigated have been migrated.
Telemetry's page navigation event now also hooks into these events to generate
the telemetry record.
In order to support click handling on anchors from JavaScript, we need some hook
from the page/session to the CDP instance. This first phase adds notifications
in page.navigate, as well as a primitive notification hook to the session.
CDP's existing Page.navigate uses this new notifiation system.
