refactor: add copy method to CompressionEngine and update mode_diff to use it

src/compression/gzip.rs

@@ -40,6 +40,13 @@ impl CompressionEngine for CompressionEngineGZip {
 
         Ok(Box::new(AutoFinishGzEncoder::new(gzip_write)))
     }
+
+    fn copy(&self, file_path: PathBuf, writer: &mut dyn Write) -> Result<()> {
+        let mut reader = self.open(file_path)?;
+        io::copy(&mut reader, writer)?;
+        writer.flush()?;
+        Ok(())
+    }
 }
 
 pub struct AutoFinishGzEncoder<W: Write> {