Support use cases where you want to provision STS tokens
using Vault, but, you need to call AWS APIs that are blocked
for federated tokens. For example, STS federated tokens cannot
invoke IAM APIs, such as Terraform scripts containing
`aws_iam_*` resources.
The new STS path allows for obtaining the same credentials that you would get
from the AWS "creds" path, except it will also provide a security token, and
will not have an annoyingly long propagation time before returning to the user.
