The user should rebuild the vtxo tree transactions instead of rely on the ASP

[louisinger]

For security reasons, the users and the ASP shouldn't exchange PSBTs but instead share common data letting to rebuild the transactions.

It means change in the API:

• CreatePaymentResponse : raw signature instead of signed_redeem_tx.
• FinalizePaymentRequest : list of raw signatures (mapped to outpoints) instead of signed_forfeit_txs and signed_round_tx.
• RoundFinalizationEvent : remove forfeits_txs and congestion_tree, replace by VTXOs leaves + remove connectors list (can be recomputed from round transaction).