Change `WalletSource::sign_tx` to `sign_psbt`

[benthecarman]

Currently WalletSource::sign_tx gives you a transaction with a signed anchor output. This is a problem if the user has any taproot inputs since taproot signatures commit to all the input's previous output, they will have their own previous output but likely not have the one for the anchor output without looking it up. It would be more ideal if a PSBT was provided with all the fields provided for the anchor, this way the user can sign it much easier.