Tweak PSBT signing for bindings compatibility

In bindings we can't practically pass a mutable PSBT, and instead
need to pass an owned transaction and have the sign method return a
signed copy. We do this here for all build modes as its not a
material API change for Rust users.

Author: TheBlueMatt

lightning/src/sign/mod.rs

@@ -1312,7 +1312,7 @@ impl KeysManager {
 	///
 	/// May panic if the [`SpendableOutputDescriptor`]s were not generated by channels which used
 	/// this [`KeysManager`] or one of the [`InMemorySigner`] created by this [`KeysManager`].
-	pub fn sign_spendable_outputs_psbt<C: Signing>(&self, descriptors: &[&SpendableOutputDescriptor], psbt: &mut PartiallySignedTransaction, secp_ctx: &Secp256k1<C>) -> Result<(), ()> {
+	pub fn sign_spendable_outputs_psbt<C: Signing>(&self, descriptors: &[&SpendableOutputDescriptor], mut psbt: PartiallySignedTransaction, secp_ctx: &Secp256k1<C>) -> Result<PartiallySignedTransaction, ()> {
 		let mut keys_cache: Option<(InMemorySigner, [u8; 32])> = None;
 		for outp in descriptors {
 			match outp {
@@ -1374,7 +1374,7 @@ impl KeysManager {
 			}
 		}
 
-		Ok(())
+		Ok(psbt)
 	}
 
 	/// Creates a [`Transaction`] which spends the given descriptors to the given outputs, plus an
@@ -1396,7 +1396,7 @@ impl KeysManager {
 	/// this [`KeysManager`] or one of the [`InMemorySigner`] created by this [`KeysManager`].
 	pub fn spend_spendable_outputs<C: Signing>(&self, descriptors: &[&SpendableOutputDescriptor], outputs: Vec<TxOut>, change_destination_script: Script, feerate_sat_per_1000_weight: u32, locktime: Option<PackedLockTime>, secp_ctx: &Secp256k1<C>) -> Result<Transaction, ()> {
 		let (mut psbt, expected_max_weight) = SpendableOutputDescriptor::create_spendable_outputs_psbt(descriptors, outputs, change_destination_script, feerate_sat_per_1000_weight, locktime)?;
-		self.sign_spendable_outputs_psbt(descriptors, &mut psbt, secp_ctx)?;
+		psbt = self.sign_spendable_outputs_psbt(descriptors, psbt, secp_ctx)?;
 
 		let spend_tx = psbt.extract_tx();