Move transaction history and wallet current state out of SpWallet

[Sosthene00]

Currently we have a Rust ad hoc struct that stores all the available and spent outputs along with transaction history and the sp client, and we store it as a stringified json in secure storage. I think that it would be better to totally move it out of the rust into the flutter part. That would allows us to store that kind of information in a structured way, even in a simple relational database, leaving only the most sensitive information in secure storage. That would also allow for a cleaner api with the rust part, instead of passing the whole stringify wallet each time we need to interact with the wallet

[cygnet3]

I think this is pretty important for backups, so I added this to that milestone