feat: add db-channel-uri config to allow reload using NOTIFY for read replicas

[laurenceisla]

Should close #3414.

Applies to the cases where a PostgREST instance connects to a Read Replica. Then, the db-channel-uri config can be used to set a connection to the Primary DB, allowing PostgREST to reload when an Event Trigger is executed in that Primary DB.

[laurenceisla]

This needs a debounce for when the connection is lost or the db-channel-uri has an error.

Since this should apply only when db-channel-uri is set, perhaps it should run conditionally, like:

Suggested change

	connectionWorker appState
	-- TODO: debounce
	-- retry the listener
	listener appState conf
	when (isNothing dbChannel) $
	connectionWorker appState
	-- TODO: debounce
	-- retry the listener
	listener appState conf

[wolfgangwalther]

I don't like this approach and suggested an alternative here: #3414 (comment)

[steve-chavez]

Let's try Wolfgang's approach.