support temporary credentials

## Problem
AWS Toolkit does not have a UI prompt or a guided credential_process to prompt user for temporary credentials — i. e. the ones that use AWS_SESSION_TOKEN in addition to AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY

## Expected behavior
user can input an aws_session_token for temp credentials

Start by tracing the AWS credential input and credential_process flows and searching for existing handling of AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, and AWS_SESSION_TOKEN. Done means the UI or guided credential flow accepts temporary credentials including aws_session_token and uses them successfully.

Written by the indexing model from the issue text.