Add IAM tagging support for iam_user roles in AWS secret engine

[Valarissa]

This PR seeks to introduce the ability to add tags to user credentials generated by the AWS secret engine.

Details in usage are provided in the docs that have been updated with this feature, however, for ease of use, the following is an example of adding two tags to a role:

vault write aws/roles/my-role \
credential_type=iam_user \
iam_tags=test_key=test_value \
iam_tags=test_key2=test_value2 \
policy_document=-<<EOF
{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Action": "ec2:*",
      "Resource": "*"
    }
  ]
}
EOF