[Abstraction] Determine a dependency versioning strategy for event-bus implementations

The edx_event_bus_kafka dependency will be specified and pinned in a file outside of each IDA that uses it, since openedx-events will load the implementation by looking up a module reference in the IDA's config. This means that dependency version changes on the library will not be versioned with the IDA (i.e. in the requirements/base.txt files) and we'll need some sort of expand-contract approach to API changes.

For example, we might need to do something like this:

1. Add a new KafkaProducer class in event-bus-kafka and delegate the existing send_to_event_bus to use it
2. Switch the producer code in Studio to use the new class
3. Delete the send_to_event_bus function from event-bus-kafka

Every master-branch deployer would need to do this in lockstep. (Maybe that's just 2U?) For Open edX releases we might be able to get away with just saying "use the release tag".

Acceptance Criteria

• Design a strategy that will work for deployers in general
• Document how-to for these upgrades in appropriate places.

Begin with the IDA requirements/base.txt files and the openedx-events module-reference loading path described in the issue. Define a dependency upgrade strategy that works for deployers and releases, then document the upgrade steps in the appropriate project locations. Done means both acceptance criteria are addressed.