Take metadata into account in asset sync comparison

Could not find it being accounted for hear: https://github.com/aws/aws-cli/blob/4ff0cbacbac69a21d4dd701921fe0759cf7852ed/awscli/customizations/s3/syncstrategy/base.py

And in behavior seen between invoking the sync and on s3, it doesn't look like it is being accounted for.
e.g. If you execute sync with `--cache-control max-age=1234`, then execute sync again with the same asset(s) with `--cache-control no-cache`. The metadata doesn't trigger an update.

Expected: Metadata is used in asset comparison to determine whether a sync should occur.