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

src/digest_engine.rs

@@ -32,6 +32,9 @@ pub trait DigestEngine {
     }
     fn create(&self) -> Result<Box<dyn Write>>;
     fn finalize(&mut self) -> io::Result<String>;
+    
+    // Update the digest with new data
+    fn update(&mut self, data: &[u8]);
 }
 
 lazy_static! {