feature: Support IAM role instead of access / secret keys to use AWS S3 bucket

Hi, team.
Some companies force users not to directly own the access / secret keys for AWS S3 bucket. In this case, users can create an AWS IAM role and own the permission for S3 bucket. So it would be good if jnotebook_reader also supports this scenario.

Start by locating the existing AWS S3 credential and bucket configuration in the repository. Compare the current access-key flow with AWS IAM role authentication, then verify that a notebook can be read from an IAM-authorized S3 bucket without directly supplied access or secret keys.

Written by the indexing model from the issue text.