Callback to listen the progress for sending/receiving data.
count is the length of the bytes have been sent/received.
total is the content length of the response/request body.
1.When receiving data:
total is the request body length.
2.When receiving data:
total will be -1 if the size of the response body is not known in advance,
for example: response data is compressed with gzip or no content-length header.
typedef ProgressCallback = void Function(int count, int total);