Expose LISTEN pubsub functionality

[bgentry]

We already have a robust LISTEN/NOTIFY implementation internally. It would potentially be useful for a variety of user-space coordination activities.

Expose it by wrapping our internal notifier package in a minimal public API. Ensure that we validate topic names in the process.

TODO: add test coverage

[geekodour]

Will adding this feature allow us to be notified of a completion of a job, I want to send a realtime update of the status of the job to the fronetend and to other parts of the backend. Think this should be already possible right? Does this add something else?

EDIT: https://riverqueue.com/docs/subscriptions

I completely missed this^ apologies

[brandur]

@geekodour Yeah, exactly. The subscriber interface is the way to go for that.