Bugfix: solve "Invalid signature" error on AWS IAM Roles Anywhere

[amitlicht]

Description

This fixes a bug in AWS IAM Roles Anywhere, caused due to a regression in AWS API.
Prior to this fix, an empty X-Amz-X509-Chain header was added to AWS Roles Anywhere CreateSession request. This recently started failing the request, due to a change in the AWS API.
This happens when the slice of intermediate CAs is empty. However, by passing a nil slice into the AWS roles anywhere signing helper, we make it avoid adding the empty header, which solves the issue.

References

• Relevant line in rolesanywhere aws_signing_helper: https://github.com/aws/rolesanywhere-credential-helper/blob/v1.0.3/aws_signing_helper/signer.go#L213

Testing

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• This change adds test coverage for new/changed/fixed functionality

Checklist

• I have added documentation for new/changed functionality in this PR and in github.com/otterize/docs