Fix string setter thunk to use PtrAndLen instead of two args.

PiperOrigin-RevId: 565118836
diff --git a/rust/internal.rs b/rust/internal.rs
index 5f62b15..1e0f536 100644
--- a/rust/internal.rs
+++ b/rust/internal.rs
@@ -97,3 +97,9 @@
         }
     }
 }
+
+impl From<&[u8]> for PtrAndLen {
+    fn from(slice: &[u8]) -> Self {
+        Self { ptr: slice.as_ptr(), len: slice.len() }
+    }
+}