Allow arbitrary length path_id in encrypted_data

BOLT 4 allows the path_id in encrypted_data to be of an arbitrary
length, but the code currently only allows 32 bytes. Update it to match
the specification.

lightning/src/blinded_path/message.rs

@@ -29,7 +29,7 @@ pub(crate) struct ReceiveTlvs {
 	/// If `path_id` is `Some`, it is used to identify the blinded path that this onion message is
 	/// sending to. This is useful for receivers to check that said blinded path is being used in
 	/// the right context.
-	pub(crate) path_id: Option<[u8; 32]>,
+	pub(crate) path_id: Option<Vec<u8>>,
 }
 
 impl Writeable for ForwardTlvs {
@@ -45,9 +45,12 @@ impl Writeable for ForwardTlvs {
 
 impl Writeable for ReceiveTlvs {
 	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
+		const EMPTY_PATH_ID: &Vec<u8> = &vec![];
+		let path_id = self.path_id.as_ref().unwrap_or(EMPTY_PATH_ID);
+
 		// TODO: write padding
 		encode_tlv_stream!(writer, {
-			(6, self.path_id, option),
+			(6, *path_id, optional_vec),
 		});
 		Ok(())
 	}

lightning/src/blinded_path/mod.rs

@@ -138,4 +138,3 @@ impl_writeable!(BlindedHop, {
 	blinded_node_id,
 	encrypted_payload
 });
-

lightning/src/onion_message/messenger.rs

@@ -337,7 +337,7 @@ where
 	}
 
 	fn respond_with_onion_message<T: CustomOnionMessageContents>(
-		&self, response: OnionMessageContents<T>, path_id: Option<[u8; 32]>,
+		&self, response: OnionMessageContents<T>, path_id: Option<Vec<u8>>,
 		reply_path: Option<BlindedPath>
 	) {
 		let sender = match self.node_signer.get_node_id(Recipient::Node) {

lightning/src/onion_message/packet.rs

@@ -280,7 +280,7 @@ impl Readable for ControlTlvs {
 			(1, _padding, option),
 			(2, _short_channel_id, option),
 			(4, next_node_id, option),
-			(6, path_id, option),
+			(6, path_id, optional_vec),
 			(8, next_blinding_override, option),
 		});
 		let _padding: Option<Padding> = _padding;