Update demos

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

demos/src/base64_demo.c

@@ -0,0 +1,56 @@
+/*
+ *  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/base64.h>
+#include <gmssl/error.h>
+#include <gmssl/rand.h>
+
+int main(void)
+{
+	BASE64_CTX ctx;
+
+	uint8_t buf[200];
+	char base64[400] = {0};
+	uint8_t *in = buf;
+	uint8_t *out = (uint8_t *)base64;
+	int len;
+	int i;
+	int inlen = 47;
+
+	rand_bytes(buf, sizeof(buf));
+
+	base64_encode_init(&ctx);
+
+	base64_encode_update(&ctx, in, inlen, out, &len);
+	out += len;
+	in += inlen;
+	printf("1 %s\n", base64);
+
+	base64_encode_update(&ctx, in, inlen, out, &len);
+	out += len;
+	in += inlen;
+	printf("2 %s\n", base64);
+
+	base64_encode_update(&ctx, in, 30, out, &len);
+	out += len;
+	in += 48;
+	printf("3 %s\n", base64);
+
+	base64_encode_update(&ctx, in, 30, out, &len);
+	out += len;
+	in += 48;
+	printf("4 %s\n", base64);
+
+	return 0;
+
+}