This builds upon the new EventStreamHttpResponse class to easily

enable APIs to stream events and results to callers using Server-Sent

Events.
WebAPIResource handlers can now return a generator or generator

function that yields messages for the event stream. These will be

serialized one-by-one using the same API response serialization logic we

otherwise use for the API. Callers can parse those as they come in,

utilizing a X-Event-Data-Mimetype header to know the resource type for

any "message" events (the default when event isn't specified) to

intelligently handle parsing.
This can be used for HTTP GET/POST/PUT requests, making it possible to

stream progress notification or build a message bus using the API.

Unit tests pass.
Tested this with an EventSource in JavaScript, verifying that events

were properly coming in and could be parsed correctly.