Send RTPS header and payload in one stream

This avoids assembling the header and payload on the receiver side

msg/templates/urtps/microRTPS_transport.h

@@ -46,6 +46,8 @@ public:
 	virtual uint8_t close() {return 0;}
 	ssize_t read(uint8_t *topic_ID, char out_buffer[], size_t buffer_len);
 	ssize_t write(const uint8_t topic_ID, char buffer[], size_t length);
+	/* Get the Length of struct Header to make headroom for the size of struct Header alongwith payload*/
+	ssize_t get_header_length();
 
 protected:
 	virtual ssize_t node_read(void *buffer, size_t len) = 0;