Add support for database as queue backend

[prabirshrestha]

Would be good if database backends were supported.

I have written one for surrealdb and would be happy to contribute if you are open. https://github.com/prabirshrestha/mq/.

Other database I'm interested in are sqlite and Postgres.

[tasn]

Hey @prabirshrestha! Sqlite and postgres: yes, we would love that, thanks a lot. Looking forward to seeing your PRs!

As for surrealdb, let us think about it. We are the one that will end up needing to maintain it, and I want to make sure we don't take on something that's not as widely used.

[prabirshrestha]

Was working on surrealdb backend by following other samples, but seems like queue mod is private to the crate so queue::{Acker, Delivery, QueueBackend} doesn't seem available.

[tasn]

@svix-jplatte, can you please take a look at the above report?

[svix-jplatte]

Yeah, that is expected. It's not currently supported to define a queue backend outside of omniqueue. I'm not sure how hard it is to change that, it was already the case when I started working on omniqueue (even though the docs actually claimed that it's possible at that point).