Update terraform-aws-spacelift-workerpool-on-ec2 to support PermissionsBoundaries

https://github.com/spacelift-io/terraform-aws-spacelift-workerpool-on-ec2/blob/d6ead175668024308f2eecfc2952246fe7f084e9/iam.tf#L109-L127

The IAM roles within terraform-aws-spacelift-workerpool-on-ec2 do not support PermissionsBoundaries. In some AWS accounts, this is a hard requirement. Doing a pass-through variable for iam role permissions boundaries is a standard practice (example). Right now the module has no workaround to allow for autoscaling with permissions boundaries.