Provide a better way to extract headers from a CloudEvent

[lance]

Currently, in order to extract headers from a CloudEvent, you have to first create an HTTPEmitter.

const ce = new CloudEvent({ type: 'example.event', source: 'https://example.event.source' });
const emitter = new HTTPEmitter({ url: 'http://bogus.url' });
const headers = emitter.headers();

Not ideal... you have to create a new object for no real reason. And on top of that, you have to supply it with a URL since the emitter ctor will throw without one.

Something better might be...

const { headers /* other exports such as HTTPEmitter */ } = require('cloudevents-sdk/bindings/http');
 
const ce = new CloudEvent({ type: 'example.event', source: 'https://example.event.source' });
const headers = headers(ce);