Request to support US3

[JohnNiang]

In order to achieve better transmission performance, only 8MB parts are supported by default. Please see https://docs.ucloud.cn/ufile/s3/s3_introduction?id=s3%e7%9a%84%e5%88%86%e7%89%87%e5%a4%a7%e5%b0%8f%e8%af%b4%e6%98%8e for more.

But Halo S3 plugin only supports 5MB parts by default. So I request to support 8MB parts only for US3 instead of all vendors.

/kind feature