fix: create sha2_digest module for digest engine

src/digest_engine/sha2_digest.rs

@@ -0,0 +1,31 @@
+use std::io;
+use sha2::{Digest, Sha256};
+
+#[derive(Debug, Eq, PartialEq, Clone, Default)]
+pub struct DigestEngineSha256 {
+    hasher: Sha256,
+}
+
+impl DigestEngineSha256 {
+    pub fn new() -> DigestEngineSha256 {
+        DigestEngineSha256 { hasher: Sha256::new() }
+    }
+}
+
+use crate::digest_engine::DigestEngine;
+
+impl DigestEngine for DigestEngineSha256 {
+    fn create(&self) -> Box<dyn DigestEngine> {
+        Box::new(Self::new())
+    }
+
+    fn update(&mut self, data: &[u8]) -> io::Result<()> {
+        self.hasher.update(data);
+        Ok(())
+    }
+
+    fn finalize(&mut self) -> io::Result<String> {
+        let result = self.hasher.finalize();
+        Ok(format!("{:x}", result))
+    }
+}