Update demos

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

demos/src/asn1_oid_to_der_demo.c

@@ -0,0 +1,49 @@
+/*
+ *  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/oid.h>
+#include <gmssl/asn1.h>
+
+
+int main(int argc, char **argv)
+{
+	int ret = -1;
+	uint32_t oid[] = {1,2,156,10197,1};
+	uint8_t buf[64];
+	uint8_t *p;
+	size_t len;
+	size_t i;
+
+	p = buf;
+	len = 0;
+	if (asn1_object_identifier_to_der(oid, sizeof(oid)/sizeof(oid[0]), &p, &len) != 1) {
+		fprintf(stderr, "asn1_object_identifier_to_der() error\n");
+		goto err;
+	}
+
+	printf("oid: ");
+	for (i = 0; i < sizeof(oid)/sizeof(oid[0]); i++) {
+		printf("%u ", oid[i]);
+	}
+	printf("\n");
+
+	printf("der: ");
+	for (i = 0; i < len; i++) {
+		printf("%02x ", buf[i]);
+	}
+	printf("\n");
+
+	ret = 0;
+err:
+	return ret;
+}