[PACKET_IO]format api style

sdk/include/packet.h

@@ -1,3 +1,16 @@
 #pragma once
 
-struct stellar_packet;
+#include <stddef.h>
+
+struct stellar_packet;
+
+/**
+ * @brief get stellar_packet's ctrlzone 
+ * @note ctrlzone's memory is 64 bytes, do not exceed it
+ */
+char *get_stellar_packet_ctrlzone(struct stellar_packet *p, size_t *ctrlzone_len);
+
+/**
+ * @brief get stellar_packet's data pointer
+ */
+char *get_stellar_packet_data(struct stellar_packet *p, size_t *data_len);