Allows vault roles to be associated with IAM groups in the AWS
secrets engine, since IAM groups are a recommended way to manage
IAM user policies. IAM users generated against a vault role will
be added to the IAM Groups. For a credential type of
`assumed_role` or `federation_token`, the policies sent to the
corresponding AWS call (sts:AssumeRole or sts:GetFederation) will
be the policies from each group in `iam_groups` combined with the
`policy_document` and `policy_arns` parameters.
Co-authored-by: Jim Kalafut <jkalafut@hashicorp.com>
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