[wallet] Document about extra validation when signing transactions including foreign utxos

[ValuedMammal]

The wallet should check that a foreign utxo is indeed foreign before proceeding to build a transaction with it.

See a related discord comment

https://discord.com/channels/753336465005608961/978744259693916230/1326206434144288821

[nymius]

I think this check is necessary. We could filter out foreign utxos when their associated outpoints are found in the tx graph of the wallet.

[nymius]

@ValuedMammal we were discussing with @evanlinjin about this change and I think we need to agree first in definition of foreign. Let's consider the following scenario:

• An out of sync (on purpose) wallet that generates addresses (so it index new spks) but doesn't contain any transaction. So there might be UTxOs around there that the wallet can sign, but doesn't know about them yet. At the time of creating a new transaction, those UTxOs are given to the wallet as foreign ones (because add_utxo will reject its addition as they aren't contained by the wallet). The current code in master will allow this behavior, and also the one exposed by the discord comment.
  We came to the conclusion that is preferably to not have the first behavior in order to avoid the security issues produced by the second one, but would like to have a third opinion about it. What do you think?

[ValuedMammal]

If there's a use case for adding local utxos via add_foreign_utxo, or if we simply can't know whether a utxo is local or foreign (as in the case of an empty wallet), then I think it's better to keep the current behavior of allowing any outpoint to be passed to add_foreign_utxo as long as we still filter duplicates in the end.

[nymius]

As discussed in today lib dev team call, the behavior explained above is something we should not constrain, and the issue mentioned in the discord chat should be controlled by the user.

This decision makes bitcoindevkit/bdk#1823 irrelevant and we should be aiming here for a documentation change. Open for grabs to anyone interested in adding this.