refactor: add update method to digest engines for consistent write interface

src/digest_engine/none.rs

@@ -19,4 +19,6 @@ impl DigestEngine for DigestEngineNone {
     fn finalize(&mut self) -> io::Result<String> {
         Ok("none".to_string())
     }
+    
+    fn update(&mut self, _data: &[u8]) {}
 }