handle s3.amazonaws.com.cn URL for s3 getter

[julian7]

AWS China endpoints are different, and the current URL detector is providing invalid results. I've updated URL detection based on the following documents:

• Generic AWS Endpoints: https://docs.amazonaws.cn/en_us/aws/latest/userguide/endpoints-arns.html
• AWS China Endpoints: https://docs.amazonaws.cn/en_us/aws/latest/userguide/endpoints-arns.html

Fixes #128

[hashicorp-cla]

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