# Structs
ChainedCredentialsProvider 存储多个 CredentialsProvider，逐个尝试直到成功获取第一个 Credentials 为止.
EnvironmentVariableCredentialProvider 从环境变量中获取 Credential.
# Interfaces
CredentialsProvider 获取 Credentials 对象的接口.
# Type aliases
Credentials 七牛鉴权类，用于生成Qbox, Qiniu, Upload签名
AK/SK可以从 https://portal.qiniu.com/user/key 获取.