FillPSBT: report number of inputs signed (or would sign)

In FillPSBT, optionally report the number of inputs we successfully
signed, as an out parameter. If "sign" is false, instead report the
number of inputs for which GetSigningProvider does not return nullptr.
(This is a potentially overbroad estimate of inputs we could sign.)

src/qt/sendcoinsdialog.cpp

@@ -392,7 +392,7 @@ void SendCoinsDialog::on_sendButton_clicked()
         CMutableTransaction mtx = CMutableTransaction{*(m_current_transaction->getWtx())};
         PartiallySignedTransaction psbtx(mtx);
         bool complete = false;
-        const TransactionError err = model->wallet().fillPSBT(SIGHASH_ALL, false /* sign */, true /* bip32derivs */, psbtx, complete);
+        const TransactionError err = model->wallet().fillPSBT(SIGHASH_ALL, false /* sign */, true /* bip32derivs */, psbtx, complete, nullptr);
         assert(!complete);
         assert(err == TransactionError::OK);
         // Serialize the PSBT

src/qt/walletmodel.cpp

@@ -536,7 +536,7 @@ bool WalletModel::bumpFee(uint256 hash, uint256& new_hash)
     if (create_psbt) {
         PartiallySignedTransaction psbtx(mtx);
         bool complete = false;
-        const TransactionError err = wallet().fillPSBT(SIGHASH_ALL, false /* sign */, true /* bip32derivs */, psbtx, complete);
+        const TransactionError err = wallet().fillPSBT(SIGHASH_ALL, false /* sign */, true /* bip32derivs */, psbtx, complete, nullptr);
         if (err != TransactionError::OK || complete) {
             QMessageBox::critical(nullptr, tr("Fee bump error"), tr("Can't draft transaction."));
             return false;