Add option to verify SHA256 hash during S3 downloads

[an-presley]

Currently, when writing data to S3, tensorstore includes the data's SHA256 hash in the x-amz-content-sha256 header. From the docs, it looks like S3 returns this hash during downloads, but tensorstore doesn't currently verify it against the downloaded data. It would be helpful if there was an option to verify it, to build confidence there is no corruption when reading tensors from S3.

[laramiel]

Something to be aware of for #114