refactor: explicitly deserialize messages in event consumer

[rgraber]

Manually deserialize messages from a Consumer instead of using the DeserializingConsumer. This is in service of eventually being able to consume events of multiple types from the same object. Left as Unreleased because it is just the first PR of a series to add the full feature.

Issue: openedx/openedx-events#78

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