NofN_xonly_key can change

[atacann]

NofN key changes whenever a verifier enters/exits the system. Currently aggregator creates the nofn key during Aggregator::new() and it stays fixed from there. We need to change it so that nofn key is calculated from currently active verifiers. Additionally store nofn keys on the db for the deposit_info so that we can retrieve which nofn key was used to sign a particular deposit. Without the nofn key we cannot reconstruct the deposit transactions during withdrawal.