Add optional name field to activity definitions

Introduce an optional `name` field in `ActivityDefinition` to allow better identification of activities. Updated test utilities and the `test_defined_activity_generator` to accommodate and validate this new field.

Author: Onyekachukwu-Nweke

simln-lib/src/defined_activity.rs

@@ -90,13 +90,15 @@ mod tests {
 
     #[test]
     fn test_defined_activity_generator() {
+        let name: String = "test_generator".to_string();
         let node = create_nodes(1, 100000);
         let node = &node.first().unwrap().0;
 
         let source = get_random_keypair();
         let payment_amt = 50;
 
         let generator = DefinedPaymentActivity::new(
+            Option::from(name),
             node.clone(),
             None,
             None,

simln-lib/src/test_utils.rs

@@ -212,6 +212,7 @@ pub fn create_activity(
     amount_msat: u64,
 ) -> ActivityDefinition {
     ActivityDefinition {
+        name: None,
         source,
         destination,
         start_secs: None,