Implement API-Level Events

[jamescarr]

There are no signals for various events that happen

This is optional as per the spec but useful for integrators.

Definition of done:

• Event Handlers are registered via the global API
• Events are emitted internally when they occur
• associated tests

[toddbaert]

There's nothing wrong with implementing the API-level events in isolation - but I would caution that doing so (separately from client events) could increase the chances that we have breaking changes in the client events implementation.

We should keep that in mind when we undertake this.