[FEATURE] S3 store backend - add possibility to access bucket assuming iam role

[ext-gvillafane]

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Changes proposed in this pull request:

• Accept role_arn attribute to be able to reach S3 buckets through assume role operation

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[talagluck]

Hi @ext-gvillafane - thank you so much for your contribution! Someone on the team will review and be in touch. In the meantime, it looks like the PR is failing a linting check. Could you please run the linters as we describe in our contribution docs and re-commit? Thanks!

[anthonyburdi]

Hi @ext-gvillafane! Thank you for this contribution. I was just trying to make the changes that @kenwade4 suggested (lint / merge develop) but unfortunately I'm not able to push to your branch. Since this PR has been outstanding for a while I will close it but if you can either make those edits or open the branch so I can make those changes we can reopen and get this change in. Thank you!