Support setting referent properties via attributes (#843)

Co-authored-by: Kenneth Loeffler <kenloef@gmail.com>

src/snapshot_middleware/project.rs

@@ -11,6 +11,7 @@ use crate::{
         InstanceContext, InstanceMetadata, InstanceSnapshot, InstigatingSource, PathIgnoreRule,
         SyncRule,
     },
+    RojoRef,
 };
 
 use super::{emit_legacy_scripts_default, snapshot_from_vfs};
@@ -279,6 +280,10 @@ pub fn snapshot_project_node(
         metadata.ignore_unknown_instances = true;
     }
 
+    if let Some(id) = &node.id {
+        metadata.specified_id = Some(RojoRef::new(id.clone()))
+    }
+
     metadata.instigating_source = Some(InstigatingSource::ProjectNode(
         project_path.to_path_buf(),
         instance_name.to_string(),