Feature: allow multiple payment backends

[asmogo]

Currently, cdk-mintd is designed to allow a single lightning backend. Expanding this to allow multiple concurrent backends would enable:

• Having multiple lightning backends for redundancy.
• Unit specific backends: sat - usd - eur in a single mint

Solution to this could be allowing a arrays of [ln] backends including multiple [database] blocks

[asmogo]

we should add this before #1127

[thesimplekid]

Having multiple lightning backends for redundancy.

This is a limitation of cdk not mintd. cdk mint has the payment processors as a hashmap of the unit/method pair being the key (

cdk/crates/cdk/src/mint/mod.rs

Line 70 in 54a0b54

payment_processors: Arc<HashMap<PaymentProcessorKey, DynMintPayment>>,

). So If we were to support multiple backends for a unit method pair we would have to change this and likely implement some method of selecting which one to use.

Unit specific backends: sat - usd - eur in a single mint

This is a limitation of mintd config and already supported by cdk, so we should add support for this and should not be to difficult.

I am not against supporting multiple backends for a single unit/method paid but I think it would be a bigger refactor and we should split them.

Solution to this could be allowing a arrays of [ln] backends including multiple [database] blocks

Array of ln backends makes sense. But why do we need multiple database blocks?