Introduce WithPadding struct

1. The struct acts as a wrapper for the tlvs.
2. This stuct help us to know the size of the largest packet at the time
   of writing and hence calculate the padding size appropriately before
writing them.

Author: shaavan

lightning/src/blinded_path/utils.rs

@@ -177,3 +177,27 @@ impl Writeable for Padding {
 		Ok(())
 	}
 }
+
+
+/// A wrapper struct that stores the largest packet size for a [`BlindedPath`].
+/// This helps us calculate the appropriate padding size for the tlvs when writing them.
+pub(super) struct WithPadding<T: Writeable> {
+	/// Length of the packet with the largest size in the [`BlindedPath`].
+	pub(super) max_length: usize,
+	/// The current packet's TLVs.
+	pub(super) tlvs: T,
+}
+
+impl<T: Writeable> Writeable for WithPadding<T> {
+	fn write<W: Writer>(&self, writer: &mut W) -> Result<(), io::Error> {
+		let length = self.max_length.checked_sub(self.tlvs.serialized_length());
+		debug_assert!(length.is_some(), "Size of this packet should not be larger than the size of largest packet.");
+		let padding = Some(Padding::new(length.unwrap()));
+
+		encode_tlv_stream!(writer, {
+			(1, padding, option)
+		});
+
+		self.tlvs.write(writer)
+	}
+}