Feature/1574 flag aws accounts with iam users

[alexanderdean111]

Description

Added rule to flag AWS accounts that have IAM users.

The presence of IAM Users is not necessarily a problem: Users are a totally legitimate way to achieve various goals. However, Roles are considered a better way to achieve most of those goals in most contexts. In an account whose external access is managed through some SSO system, there should normally be no Users. IAM Users in such an environment are, as often than not, either old and forgotten or created by some dev looking for a shortcut.

Fixes #1574

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works (optional)
• New and existing unit tests pass locally with my changes