分片上传素材（预上传）
POST {{base_url}}/drive/v1/medias/upload_prepare
发送初始化请求获取上传事务ID和分块策略，目前是以4MB大小进行定长分片。
参考接口文档：分片上传素材（预上传）
Request Body
{"file_name"=>"katong.jpeg", "parent_node"=>"doccng7PzY****YFAZhK0gJh", "parent_type"=>"doc_image", "size"=>41231}
RESPONSES
status: OK
{"code":0,"data":{"block_num":1,"block_size":20971520,"upload_id":"7038565129780264961"},"msg":"Success"}