WIP - Include outcome in close

[arminsabouri]

For @zealsham, related to https://github.com/payjoin/rust-payjoin/pull/1164 . When we we're talking about include the session outcome in close() this is kinda what I had in mind. The idea being that applications can scrutinize the outcome in close and make a decision to fallback or not. Alternatively you could create seperate types for "session closing events" and "protocol events" -- which is nice for type safety -- but seems to complicate to code.

However, this approach is a bit awkward for the application. It implies that they have a rpc client where ever they are doing db things or they will save the action to fallback and do it else where. Realistically I see applications replaying a session, seeing the status (https://github.com/payjoin/rust-payjoin/blob/2009ac0d775280188e9b574bc528ddd711cd3591/payjoin/src/core/send/v2/session.rs#L81) is fatally closed due to an error, and then fetching and broadcasting the fallback. Perhaps that is the simplest solution. cc @spacebear21

Note: this pr is just informational and meant to be closed.

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