btcwallet: add SignPsbt

lnwallet/interface.go

@@ -364,6 +364,19 @@ type WalletController interface {
 	FundPsbt(packet *psbt.Packet, minConfs int32,
 		feeRate chainfee.SatPerKWeight, account string) (int32, error)
 
+	// SignPsbt expects a partial transaction with all inputs and outputs
+	// fully declared and tries to sign all unsigned inputs that have all
+	// required fields (UTXO information, BIP32 derivation information,
+	// witness or sig scripts) set.
+	// If no error is returned, the PSBT is ready to be given to the next
+	// signer or to be finalized if lnd was the last signer.
+	//
+	// NOTE: This method only signs inputs (and only those it can sign), it
+	// does not perform any other tasks (such as coin selection, UTXO
+	// locking or input/output/fee value validation, PSBT finalization). Any
+	// input that is incomplete will be skipped.
+	SignPsbt(packet *psbt.Packet) error
+
 	// FinalizePsbt expects a partial transaction with all inputs and
 	// outputs fully declared and tries to sign all inputs that belong to
 	// the specified account. Lnd must be the last signer of the