Before this change if an --s3-profile was set which used AWS STS (eg
to assume a role) and --s3-endpoint was set then rclone would use the
value from --s3-endpoint to contact the STS server which did not work.
This fix implements an endpoint resolver which only overrides the "s3"
service if --s3-endpoint is set. It sends the "sts" service (and any
other service) to the default resolver.
Fixes#6443
See: https://forum.rclone.org/t/s3-profile-failing-when-explicit-s3-endpoint-is-present/36063/
c6b0587dc0