Role based EC2 access to S3

## Feature Request

The key parameter is required to access S3 using S3StorageDriver https://github.com/apache/libcloud/blob/7b3f55a3ac2ec8423555cd9bac3a42697ffc502c/libcloud/storage/drivers/s3.py#L1224, even if provided it fails down in the code in the absence of secret here https://github.com/apache/libcloud/blob/trunk/libcloud/common/aws.py#L313

Can you suggest the usage of your client for role-based EC2 access to S3 or the possibility to add it? (where key and secret are now known well in advance but using STS for temporary access by tokens)

Thanks!

从 libcloud/storage/drivers/s3.py 中引用的 key 要求处开始查看 S3StorageDriver 代码，然后跟踪引用行附近 libcloud/common/aws.py 中的凭据处理。确定应如何支持或记录基于角色的 EC2 访问和临时 STS 令牌；当预期用法或实现范围明确时，即视为完成。

由索引模型根据 Issue 内容生成。