* fetchs3: add configurable name
Change the default from "foo" to the basename of the s3 URL and make it
configurable.
* fetchs3: fix error on missing credentials.session_token
The session token should default to null instead of failing
* fetchs3: make use of the region argument
Set it to null if you don't want to use it
* fetchs3: prefer local build
Fetcher-types spend more time on network than CPU
