Update demos

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

demos/src/sm2_sig_from_bin_demo.c

@@ -0,0 +1,29 @@
+/*
+ *  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>
+
+
+int main(void)
+{
+	SM2_SIGNATURE sig;
+
+	// the signatue binary data might be invalid, and will not pass verification
+	uint8_t r[32] = { 1, 2, 3, };
+	uint8_t s[32] = { 4, 5, 6, };
+
+	memcpy(sig.r, r, 32);
+	memcpy(sig.s, s, 32);
+
+	return 0;
+}