fix(aws): always session from externalcreds

[dixneuf19]

Description

Fixes #463

It uses newSession function to get AWS Credentials from externalcreds instead of the old newAWSSession function which does not uses inputed config.

The inconsistent usage of this methods introduced the bug described in the attached issue.

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks. --> TBD
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• Include release notes. If this PR is backward incompatible, include a migration guide.
• Attention Grunts - if this PR adds support for a new resource, ensure the nuke_sandbox and nuke_phxdevops jobs in .circleci/config.yml have been updated with appropriate exclusions (either directly in the job or via the .circleci/nuke_config.yml file) to prevent nuking IAM roles, groups, resources, etc that are important for the test accounts.

Release Notes (draft)

Added / Removed / Updated [X].

Updated: Uses AWS config stored in externalcreds for both regional and global resources, fixing #463

Migration Guide

N/A

[dixneuf19]

Hi @denis256 @ellisonc if you are able to have a look at this PR and the issue mentioned, I think it is quite a harmful bug for users using this tool as a library !

Anyway I know that maintaining open source projects is quite work heavy, thanks anyway (especially for Terragrunt, we are strongly adopting it in our company) 😅

[hongil0316]

Thanks for spotting this bug. The code LGTM. Let me trigger the test CI pipeline.

[dixneuf19]

Hi @hongil0316 I see the test failing but I'm not sure if it related to this change or just flakiness.