SplitStreamReader: Add lookup_external_ref()

This lets you look up a ref digest from the splitstream by index
and is needed by the ostree code.

Author: alexlarsson

crates/composefs/src/splitstream.rs

@@ -532,6 +532,11 @@ impl<ObjectID: FsVerityHashValue> SplitStreamReader<ObjectID> {
         self.named_refs
     }
 
+    /// Look up the digest of an external reference by index
+    pub fn lookup_external_ref(&self, idx: usize) -> Option<&ObjectID> {
+        return self.object_refs.get(idx);
+    }
+
     fn ensure_chunk(
         &mut self,
         eof_ok: bool,