Introduce BlindedPathParams

1. Added a new struct called `BlindedPathParams` to encapsulate multiple
   properties that define a blinded path.
2. These properties include the number of paths, the compactness of the path,
   and the number of hops.
3. This struct provides a systematic way for users to specify these properties
   before creating a blinded path, offering more flexibility and control.
4. In this version, the struct supports two properties:
   - The number of paths to generate.
   - Whether the path should be compact or not.
5. The ability to specify the number of hops will be introduced in a future update.

lightning/src/onion_message/messenger.rs

@@ -417,6 +417,22 @@ pub struct PendingOnionMessage<T: OnionMessageContents> {
 	pub reply_path: Option<BlindedMessagePath>,
 }
 
+/// A placeholder value used for paths within the codebase.
+///
+/// PATHS_PLACEHOLDER` is temporarily used as a default value in situations
+/// where a path index is required but has not yet been assigned or initialized.
+pub const PATHS_PLACEHOLDER: usize = 0;
+
+/// Represents the types of [`BlindedPath`] that can be created.
+///
+/// This struct allows to specify the properties of a Blinded Path before creating it.
+pub struct BlindedPathParams {
+	/// Number of [`BlindedPath`]s to be created.
+	pub paths: usize,
+	/// Whether the created [`BlindedPath`]s are full-length or compact.
+	pub is_compact: bool,
+}
+
 #[cfg(c_bindings)]
 /// An [`OnionMessage`] for [`OnionMessenger`] to send.
 ///