feat: add sm2 encryption mode

examples/encrypt_and_decrypt.rs

@@ -0,0 +1,18 @@
+use libsm::sm2::encrypt::{DecryptCtx, EncryptCtx, Sm2EncryptionMode};
+use libsm::sm2::signature::SigCtx;
+
+fn main() {
+    let msg = "hello world".as_bytes();
+    let klen = msg.len();
+    let ctx = SigCtx::new();
+    let (pk_b, sk_b) = ctx.new_keypair().unwrap();
+
+    let encrypt_ctx = EncryptCtx::new(klen, pk_b);
+    let cipher = encrypt_ctx.encrypt(msg, Sm2EncryptionMode::C1C3C2).unwrap();
+
+    let decrypt_ctx = DecryptCtx::new(klen, sk_b);
+    let plain = decrypt_ctx.decrypt(&cipher, Sm2EncryptionMode::C1C3C2).unwrap();
+
+    println!("{}", hex::encode(&cipher));
+    println!("{}", String::from_utf8_lossy(&plain));
+}