CE timestamps should be returned as Time objects, not strings

[ace-n]

As requested by @grayside in this PR.

[grayside]

To clarify, the CloudEvent implementation in the Functions Framework currently returns a string timestamp. It would be useful if it returned a structured time object.

[ace-n]

cc @bshaffer

[bshaffer]

What is the format used by CloudEvents to convert to and from?

[grant]

This is an example time attribute HTTP header:

ce-time: "2020-04-07T17:57:33.647Z"

https://github.com/cloudevents/spec/blob/v1.0/spec.md#time
https://tools.ietf.org/html/rfc3339

Maybe DateTime::createFromFormat the property? I assume a type change would be a breaking change?