Add gateway-managed AWS STS credential refresh (provider-v2, #1576). The
gateway calls sts:AssumeRole and writes three short-lived credentials
(AWS_ACCESS_KEY_ID, AWS_SECRET_ACCESS_KEY, AWS_SESSION_TOKEN) to the
provider record; the proxy re-signs requests with SigV4. Adds the aws and
aws-s3 provider profiles and a declarative multi-output refresh model
(additional_outputs) so one AssumeRole co-mints all three credentials.
Signed-off-by: Russell Bryant <rbryant@redhat.com>
