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/onion_message/messenger.rs

@@ -432,6 +432,14 @@ pub enum MessageSendInstructions {
 	},
 }
 
+/// Represents the types of [`BlindedMessagePath`] that can be created.
+pub enum BlindedPathType {
+	/// Whether the created [`BlindedMessagePath`]s are compact.
+	Compact,
+	/// Whether the created [`BlindedMessagePath`]s are full-length.
+	Full,
+}
+
 /// A trait defining behavior for routing an [`OnionMessage`].
 pub trait MessageRouter {
 	/// Returns a route for sending an [`OnionMessage`] to the given [`Destination`].