backend/s3: Remove the requirement for region

[Xuanwo]

export S3_STORAGE_ENDPOINT_URL=https://s3.amazonaws.com

It's buggy now.

The behavior between keeping S3_STORAGE_ENDPOINT_URL empty and setting S3_STORAGE_ENDPOINT_URL=https://s3.amazonaws.com/ is different.

• S3_STORAGE_ENDPOINT_URL="": s3 SDK will use format!("s3.{}.amazonaws.com", region) to construct the correct endpoint.
• S3_STORAGE_ENDPOINT_URL=https://s3.amazonaws.com/: s3 SDK will take this input as static, and can't handle the region correctly.

I'm considering removing the region option so that:

• For AWS S3, we can always use https://s3.amazonaws.com/ and detect the region for users.
• For other services, we can let user input the correct endpoint like http://127.0.0.1:9000

Originally posted by @Xuanwo in #54 (comment)

[Xuanwo]

Implemented in #59