[DEVEX-2399]: Make errored queues configurable

[emilianobovetti]

Currently Helper.declare/2 creates a dead letter queue when the arguments contains a tuple like {"x-dead-letter-exchange", :longstr, dle}.
Since rabbit 4.0 will remove queue mirroring for classic queues, many queues need to be migrated to quorum. However declare/2 will always use the default type for dead letters, so we need a mechanism to change their types.
Ideally I'd like to refactor the Helper module, as its functions have a complex interface that has proven difficult to maintain and does not consistently behave as expected.
Unfortunately, many applications depend on declare/2 to setup both the queue and its dead letter. When {"x-queue-type", :longstr, "quorum"} is passed in the arguments amqpx will create a quorum queue and a classic dead letter, which is likely not the desired behavior.
The current plan is to use the same x-queue-type for both the queue and its dead letter. This change will be introduced in amqpx 7.0 as a breaking update, aiming to minimize the number of applications with mismatched queue types for dead letters.

ref: https://prima-assicurazioni-spa.myjetbrains.com/youtrack/issue/DEVEX-2399

[MaeIsBad]

Should there be a x-dead-letter-queue-type option, to allow overwriting the default behavior?

[MaeIsBad]

When is this a nested array?

[emilianobovetti]

here declare creates a nested string list

amqpx/lib/amqp/helper.ex

Line 104 in f9ee7dc

original_routing_keys = Enum.map(exchanges, & &1.routing_keys)

we should probably use a flat_map and remove the flatten

[emilianobovetti]

Should there be a x-dead-letter-queue-type option, to allow overwriting the default behavior?

I've briefly looked at other rabbit/amqp clients, it seems none of them automatically create dead letter queues as declare does
I think that letting applications to manage the declaration of their own resources is far simpler and already supported (like manually calling Helper.setup_dead_lettering and Helper.setup_queue has less magic involved)