Allow reading datasets from private s3 bucket

[katxiao]

Add functionality to download datasets from a private s3 bucket.

• Add aws_key and aws_secret arguments to run function and use them on the download dataset.
• boto3 needs to be used instead of urllib.
• Also adapt unit tests as necessary if any of them fail.

✅ Verified that e2e flow works when pulling data from a public s3 bucket
✅ Verified that e2e flow works when pulling data from a private s3 bucket

[CLAassistant]

All committers have signed the CLA.

[csala]

The implementation looks good, but I think that we can get rid of the old urllib approach and not require the user to pass the bucket name including the s3:// prefix on it.

[csala]

This looks almost ready, but there is a situation that should be fixed to allow system-wide credentials to be used.

[csala]

Just one last code style change (line break after if/else block) and this is ready to go :-)

[csala]

Looks good!