Add support for session token

[thomasthiebaud]

Improve/fix aws authentication. From aws4 documentation

Your AWS credentials (which can be found in your AWS console) can be specified in one of two ways:

As the second argument, like this:

  aws4.sign(requestOptions, {
    secretAccessKey: "<your-secret-access-key>",
    accessKeyId: "<your-access-key-id>",
    sessionToken: "<your-session-token>"
  })

From process.env, such as this:

  export AWS_SECRET_ACCESS_KEY="<your-secret-access-key>"
  export AWS_ACCESS_KEY_ID="<your-access-key-id>"
  export AWS_SESSION_TOKEN="<your-session-token>"

(will also use AWS_ACCESS_KEY and AWS_SECRET_KEY if available)

The sessionToken property and AWS_SESSION_TOKEN environment variable are optional for signing with IAM STS temporary credentials.