feat: Add IAM role output

[mmclane]

Description

Adds an output for IAM role arn and updates the README.md to reflect this new output.

Motivation and Context

This allows you to reference the role used by the module. Since the module might create a new role you don't know what the new role's ARN is unless you look it up in some other way. This makes it easy to reference.

Breaking Changes

No, it shouldn't break anything.

How Has This Been Tested?

• [x] I have updated at least one of the examples/* to demonstrate and validate my change(s)
• [x] I have tested and validated these changes using one or more of the provided examples/* projects
• [x] I have executed pre-commit run -a on my pull request

[bryantbiggs]

thanks - we should have exported this from the beginning. Can we copy this https://github.com/terraform-aws-modules/terraform-aws-ecs/blob/f2befd85b08c588ab2df4b515af187d2bb70c9e2/modules/cluster/outputs.tf#L57C1-L70C2 and use it here (update to match the correct name) to ensure the standard properties for the IAM role are exported

[mmclane]

thanks - we should have exported this from the beginning. Can we copy this https://github.com/terraform-aws-modules/terraform-aws-ecs/blob/f2befd85b08c588ab2df4b515af187d2bb70c9e2/modules/cluster/outputs.tf#L57C1-L70C2 and use it here (update to match the correct name) to ensure the standard properties for the IAM role are exported

I have updated the PR to include role name and unique_id to match what is being done in ECS

[antonbabenko]

This PR is included in version 3.1.0 :tada:

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