Add a redirect policy to the S3 HTTP client so X-Amz-Security-Token is
removed once a redirect chain crosses hosts. Keep stripping it on later
same-host hops in the same chain, since net/http copies headers from the
initial request for each redirect and can otherwise restore the token.
Preserve same-host redirect behavior, retain the standard redirect limit,
and add tests for cross-host, same-host, multi-hop, and redirect-loop cases.
