Update demos

To build demos, run
`cmake .. -DENABLE_DEMOS=ON`

demos/src/sdf_sign_demo.c

@@ -0,0 +1,67 @@
+/*
+ *  Copyright 2014-2023 The GmSSL Project. All Rights Reserved.
+ *
+ *  Licensed under the Apache License, Version 2.0 (the License); you may
+ *  not use this file except in compliance with the License.
+ *
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ */
+
+
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+#include <gmssl/sdf.h>
+#include <gmssl/error.h>
+
+
+int main(void)
+{
+	int ret = -1;
+	char *so_path = "libsdf_dummy.so";
+	SDF_DEVICE dev;
+
+	SDF_KEY sign_key;
+	int key_index = 0;
+	char *key_pass = "P@ssw0rd";
+
+	uint8_t dgst[32];
+	uint8_t sig[SM2_MAX_SIGNATURE_SIZE];
+	size_t siglen;
+
+	if (sdf_load_library(so_path, NULL) != 1) {
+		error_print();
+		return -1;
+	}
+
+	if (sdf_open_device(&dev) != 1) {
+		error_print();
+		goto err;
+	}
+
+	if (sdf_load_sign_key(&dev, &sign_key, key_index, key_pass) != 1) {
+		error_print();
+		goto err;
+	}
+
+	if (sdf_sign(&sign_key, dgst, sig, &siglen) != 1) {
+		error_print();
+		goto err;
+	}
+
+#if 0
+	// TODO: verify_key ...
+	if (sm2_verify(&verify_key, dgst, sig, siglen) != 1) {
+		error_print();
+		goto err;
+	}
+#endif
+
+	ret = 0;
+
+err:
+	sdf_release_key(&sign_key);
+	sdf_close_device(&dev);
+	sdf_unload_library();
+	return ret;
+}