validate aws STS policy after creation

[camerondavison]

I update an STS policy in vault that uses a federation_token and the AWS secrets backend. Unknown at the time the policy was too large for the PackedPolicySize.

I think that it would be nice if vault did not allow creating bad policies, because then it started erroring out in my clients when they tried to get the policy instead of erroring when I created it. It would be nice to have vault validate the policy, maybe by just issuing a token from AWS before it saved the update.