Support AWS auth method

[mattmahn]

Pretty much right there in the title.

I'd like to use this package in my Kubernetes workload, but the Vault server is not configured to support the Kubernetes auth method; the AWS auth method is the next best alternative.

• https://www.vaultproject.io/docs/auth/aws
• https://www.vaultproject.io/api-docs/auth/aws#login

[Lucaber]

Currently we are only implementing features we need ourselves. But we are open to pull requests.