feat: emit original headers with message

[rgraber]

Issue: openedx/openedx-events#162

Pass all EventsMetadata fields as headers when producing an event to Kafka. The consumer takes these headers and emits the signal with the original metadata, as reconstructed from the message headers.

To prevent the need for lock-step upgrades, all headers except event type and id are considered optional. If not passed, we will just use the default value in EventsMetadata. This is why _get_metadata_from_headers returns and EventsMetadata object instead of just a dictionary of kwargs to pass to send_event_with_custom_metadata.
We can make another ticket to make the other headers mandatory after and possibly rethink what _get_metadata_from_headers returns.

Merge checklist:
Check off if complete or not applicable:

• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed
• Noted any: Concerns, dependencies, deadlines, tickets, testing instructions

[timmc-edx]

A few small things, but looks good overall!