Checks for both customer managed and AWS managed policies
Examples
if (FALSE) { # aws_has_creds()
# just the policy name
aws_policy_exists("ReadOnlyAccess")
# as an ARN
aws_policy_exists("arn:aws:iam::aws:policy/ReadOnlyAccess")
# includes job-function in path
aws_policy_exists("Billing")
# includes service-role in path
aws_policy_exists("AWSCostAndUsageReportAutomationPolicy")
}