Update demos

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

demos/src/sm2_sig_to_der_demo.c

@@ -0,0 +1,40 @@
+/*
+ *  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/sm2.h>
+#include <gmssl/error.h>
+
+
+int main(void)
+{
+	SM2_KEY sm2_key;
+	uint8_t dgst[32] = {0};
+
+	SM2_SIGNATURE sig;
+	uint8_t der[SM2_MAX_SIGNATURE_SIZE];
+	uint8_t *p = der;
+	size_t derlen = 0;
+
+	sm2_key_generate(&sm2_key);
+	sm2_do_sign(&sm2_key, dgst, &sig);
+
+	if (sm2_signature_to_der(&sig, &p, &derlen) != 1) {
+		fprintf(stderr, "sm2_signature_to_der() error\n");
+		return -1;
+	}
+
+	format_bytes(stdout, 0, 0, "signature", der, derlen);
+	printf("signature length = %zu bytes\n", derlen);
+
+	return 0;
+}