feat: Add permissions boundary for IAM role

[edersonbrilhante]

Description
This PR will close issue #115. Allowing to create "aws_iam_role" "rds_enhanced_monitoring" when AWS is set to use permissions boundary in an IAM role creation.

Motivation and Context
In my case, my AWS account I am using permissions boundary to avoid creating a new role with bigger permissions. So any new role must set this boundary.
To run the resource aws_iam_role. rds_enhanced_monitoring, it is needed to set permissions boundary.

Breaking Changes
None

[edersonbrilhante]

Hey team.
Can you check is this PR makes sense?
I updated with the last changes from master

[bobbydeveaux]

Awesome @edersonbrilhante - using this now!!

[zephinzer]

Bump, having the same use case! Btw I think it would be better if the purpose of the permission_boundary was better indicated for the consumer, eg. iam_enhanced_monitoring_permission_boundary.

[edersonbrilhante]

@zephinzer I think it is a good idea. I can change, but I would like more feedback before changing.

[david-wells-1]

@edersonbrilhante personally I would keep the code as is - but you could add more detail or a link in the Readme if you want to better explain the purpose of the permission boundary. The permission boundary isn't actually for enhanced_monitoring.

[zephinzer]

@david-wells-1 it seems it's for the iam user created for the enhanced_monitoring (we're currently maintaining our own fork for this feature, would be good to have this in the main module because it seems like enterprises that are on AWS tend to impose a permissions boundary as a form of control over subaccounts)

[antonbabenko]

I believe the default value should be null for people who don't want to enforce any permissions_boundary. I have updated it.

[antonbabenko]

Thanks, everyone!

v2.18.0 has been released.

[github-actions]

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