Update demos

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

demos/src/x509_crl_download_demo.c

@@ -0,0 +1,53 @@
+/*
+ *  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/file.h>
+#include <gmssl/x509_crl.h>
+
+
+char *http_uri = "http://mscrl.microsoft.com/pki/mscorp/crl/Microsoft%20RSA%20TLS%20CA%2002.crl";
+char *file_name = "Microsoft RSA TLS CA 02.crl";
+
+int main(int argc, char **argv)
+{
+	int ret = -1;
+	uint8_t *crl = NULL;
+	size_t crllen;
+	FILE *fp = NULL;
+
+	printf("Demo - Download CRL from HTTP\n\n");
+
+	printf("    Download from %s\n", http_uri);
+
+	if (x509_crl_new_from_uri(&crl, &crllen, http_uri, strlen(http_uri)) != 1) {
+		fprintf(stderr, "x509_crl_new_from_uri() error\n");
+		goto err;
+	}
+
+	//x509_crl_print(stdout, 0, 0, "CRL", crl, crllen);
+
+	if (!(fp = fopen(file_name, "wb"))) {
+		fprintf(stderr, "fopen() error\n");
+		goto err;
+	}
+	fwrite(crl, 1, crllen, fp);
+
+	printf("    Save to %s\n", file_name);
+	printf("    Run `gmssl crlparse -in \"%s\"` to print the downloaded CRL\n", file_name);
+	printf("\n");
+
+	ret = 0;
+err:
+	if (crl) free(crl);
+	if (fp) fclose(fp);
+	return ret;
+}