Support CloudEvents

[Som-Som-CC]

Support CloudEvents with content-type application/cloudevents+json.

• Client must be able to send events with CloudEvent content-type. Probably a PostWithCT() is needed where ContentTypeCloudEvent content-type constant can be supplied.
• Server & Lambda must be able to receive event requests with CloudEvent content-type. Or anything that matches application/([^;]+\+)?json. See function isJSONContentType().