Introduce BlindedPathType

1. Introduce a new enum that allows the user to specify the type
   of Blinded Path (whether Compact or Full-Length) when creating
   a Blinded Path.
2. This PR utilizes this new enum in two ways in the upcoming commits:
   - Allows explicitly providing the type of Blinded Path to be created
     instead of relying on the absolute expiry time for offers and refunds.
   - Simplifies the Blinded Path creation process by enabling a single
     function flow for creating both types of Blinded Path.

Author: shaavan

lightning/src/ln/channelmanager.rs

@@ -221,6 +221,34 @@ pub enum PendingHTLCRouting {
 	},
 }
 
+/// Represents the types of [`BlindedMessagePath`] that can be created.
+pub enum BlindedPathType {
+	/// A compact version of [`BlindedMessagePath`].
+	///
+	/// Compact blinded paths use a [`ShortChannelId`] instead of a [`NodeId`] to represent forward nodes.
+	/// This approach drastically reduces the size of each individual forward packet but requires
+	/// knowledge of the local channel graph to find the corresponding channel for each node.
+	///
+	/// Compact blinded paths are especially useful when size is a constraint, such as when offers
+	/// and refund information must be represented in QR code form.
+	///
+	/// [`ShortChannelId`]: crate::blinded_path::message::NextMessageHop::ShortChannelId
+	/// [`NodeId`]: crate::blinded_path::message::NextMessageHop::NodeId
+	Compact,
+
+	/// A full-length version of [`BlindedMessagePath`].
+	///
+	/// Unlike compact blinded paths, full-length paths use each individual forward node's [`NodeId`] for representation.
+	/// This increases the size of each forward packet as more space is required for representation.
+	/// However, it does not require knowledge of the local channel graph to create the path.
+	///
+	/// Full-length blinded paths are useful when onion messages are communicated over the wire and size constraints are not an issue,
+	/// such as when sending a response to an onion message.
+	///
+	/// [`NodeId`]: crate::blinded_path::message::NextMessageHop::NodeId
+	Full,
+}
+
 /// Information used to forward or fail this HTLC that is being forwarded within a blinded path.
 #[derive(Clone, Copy, Debug, Hash, PartialEq, Eq)]
 pub struct BlindedForward {