Update demos

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

demos/src/sm2_private_key_parse_demo.c

@@ -0,0 +1,41 @@
+/*
+ *  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/mem.h>
+#include <gmssl/sm2.h>
+
+
+int main(void)
+{
+	SM2_KEY sm2_key;
+	char *password = "123456";
+	unsigned char buf[512];
+	unsigned char *p;
+	size_t len;
+
+	printf("Read SM2 private key file (PEM) from stdin ...\n");
+	if (sm2_private_key_info_decrypt_from_pem(&sm2_key, password, stdin) != 1) {
+		fprintf(stderr, "error\n");
+		return 1;
+	}
+
+	p = buf;
+	len = 0;
+	if (sm2_private_key_to_der(&sm2_key, &p, &len) != 1) {
+		fprintf(stderr, "error\n");
+		return 1;
+	}
+	fwrite(buf, 1, len, stdout);
+
+	gmssl_secure_clear(&sm2_key, sizeof(sm2_key));
+	return 0;
+}