Update demos

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

demos/src/sm2_ecdh_demo.c

@@ -0,0 +1,66 @@
+/*
+ *  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)
+{
+//	Alice						Bob
+
+	SM2_KEY alice_ecdhe;				SM2_KEY bob_ecdhe;
+	uint8_t alice_public[65];			uint8_t bob_public[65];
+	SM2_POINT alice_share_point;			SM2_POINT bob_share_point;
+	SM3_KDF_CTX alice_ctx;				SM3_KDF_CTX bob_ctx;
+	uint8_t alice_share_key[32];			uint8_t bob_share_key[32];
+
+
+	sm2_key_generate(&alice_ecdhe);			sm2_key_generate(&bob_ecdhe);
+
+
+	sm2_point_to_uncompressed_octets(
+		&alice_ecdhe.public_key,
+		alice_public);
+//	                              alice_public
+//	                         ====================>
+							sm2_point_to_uncompressed_octets(
+								&bob_ecdhe.public_key,
+								bob_public);
+
+							if (sm2_ecdh(&bob_ecdhe,
+								alice_public, sizeof(alice_public),
+								&bob_share_point) != 1) {
+								fprintf(stderr, "bob error\n");
+								goto err;
+							}
+							sm3_kdf_init(&bob_ctx, 32);
+							sm3_kdf_update(&bob_ctx,
+								(uint8_t *)&bob_share_point, sizeof(SM2_POINT));
+							sm3_kdf_finish(&bob_ctx, bob_share_key);
+//	                               bob_public
+//	                         <====================
+
+	if (sm2_ecdh(&alice_ecdhe,
+		bob_public, sizeof(bob_public),
+		&alice_share_point) != 1) {
+		fprintf(stderr, "Alice failed\n");
+		goto err;
+	}
+
+	sm3_kdf_init(&alice_ctx, 32);
+	sm3_kdf_update(&alice_ctx, (uint8_t *)&alice_share_point, sizeof(SM2_POINT));
+	sm3_kdf_finish(&alice_ctx, alice_share_key);
+
+err:
+	// FIXME: clean all
+	return 0;
+}