[Abstraction] Configurable loading of Event Bus consumer implementation

[timmc-edx]

We need a way to load event-bus consumer implementations based on configuration, so that IDAs aren't hardcoded to use one particular implementation. (See #87 for the producer side.)

There are a couple of ways we might go about this:

• A. Define an abstract class in openedx-events, and load an implementing class based on Django settings, the same way we do with producers.
  • See commit message on main...timmc/load-consumer for open questions about what the interface might look like. (We also need to consider whether event-replay functionality would be included in this API.)
  • Here's what the implementation might look like in event-bus-kafka: openedx/event-bus-kafka@main...timmc/consumer-api
  • openedx-events could then be registered as a Django app, and offer a "consume events" management command that reads configuration and loads the appropriate implementation.
• B. Just have every implementation make its own management command, requiring all of them to be registered in INSTALLED_APPS.
  • This moves the hardcoding from the IDA source code into the infrastructure source code.
  • Might be OK temporarily, but at some point we probably want community Helm charts or something and will not be happy with that hardcoding.
• C. Are there other options? Should we support an entry point that isn't a management command?