Refactor `bs::ops::open` to depend on traits instead of callbacks

[dhuseby]

The code here https://github.com/cryptidtech/bs/blob/main/crates/bs/src/ops/open/mod.rs should be refactored to depend on traits for the key loading and the payload signing.

Going with traits allows us to have a cleaner separation of concerns. Also, we need to embrace async since loading a key and signing a payload might be done remotely and shouldn't be blocking operations.