Access S3 URLs directly

[rgbkrk]

Similar to how the Github provider works, we should allow nbviewer to access S3 buckets and objects. If the runner of nbviewer has the appropriate access role, it could provide access to private notebooks as well.

[adammclaurin]

This is a fantastic idea

[jasonhao518]

Looking forward for this feature, potentially it can also support minio, since they are same protocol

[mnkr4844]

me too