Support different event store engines

[gap777]

Is your feature request related to a problem? Please describe.

Your docs mention the capability of specifying a different underlying store engine. Is that supported yet?

Describe the solution you'd like

To enable event propagation to non-rails clients via Redis pubsub, while still using the Rails-y usage pattern of AES

Describe alternatives you've considered

Different event buses

Additional context

[palkan]

We extracted the basic functionality (RES-independent) into another gem: https://github.com/bibendi/downstream

It support using custom message bus implementations, check it out.

P.S. The plan is to migrate active_event_store to downstream (eventually) as a RES adapter. But for now we have two almost identical libraries 🤷‍♂️