support for aws_session_token

Hi, I'm using gsutil v. 4.3.1 and there isn't any apparent support for the AWS_SESSION_TOKEN, which is a required configuration setting when working with AWS pre-signed URLs.

Use case: I'm trying to upload an object from a GCP bucket (in my account) to S3. I have been provided with an AWS pre-signed URL and corresponding credentials that include

1. AWS_ACCESS_KEY_ID
2. AWS_SECRET_ACCESS_KEY
3. AWS_SESSION_TOKEN

I have tested with these variables in the .boto file, and even in different tests as environment variables, and the response I get is "403 AccessDenied" when running my cp command that is of the form

``gsutil cp gs://mybucket/obj s3://s3bucket/obj``

Things work fine when not using a pre-signed URL.
After lots of testing, I wanted to verify that the AWS_SESSION_TOKEN is not supported, and to request for such support.

Thanks,
-Nathan