the signature of file
expires expired time (seconds)
bucket is the name of bucket used in aliyun oss
key is the object name in aliyun oss, alias the 'filepath/filename'
access aliyun need authenticated, this is the implementation refer to the official document.
req include the request headers information that use for auth.
bucket is the name of bucket used in aliyun oss
key is the object name in aliyun oss, alias the 'filepath/filename'