feat: create digest engine with Sha256/Md5/None and integrate with save mode

src/digest_engine/none.rs

@@ -0,0 +1,25 @@
+use std::io;
+use log::*;
+
+#[derive(Debug, Eq, PartialEq, Clone, Default)]
+pub struct DigestEngineNone {}
+
+impl DigestEngineNone {
+    pub fn new() -> DigestEngineNone {
+        DigestEngineNone {}
+    }
+}
+
+impl DigestEngine for DigestEngineNone {
+    fn create(&self) -> Box<dyn DigestEngine> {
+        Box::new(Self::new())
+    }
+
+    fn update(&mut self, _data: &[u8]) -> io::Result<()> {
+        Ok(())
+    }
+
+    fn finalize(&mut self) -> io::Result<String> {
+        Ok("none".to_string())
+    }
+}