Set sync conditions with a sync-if enum #10016

Describe the feature

From #599 (comment),

What about adding a single, self-documenting --sync-if flag instead of the growing number of non self-documenting options? (e.g. --size-only, --exact-timestamps. Can I apply these two at the same time? Why do I have to read the documentation / try them to figure this out?).

The --sync-if could have a list of options:
--sync-if newer-timestamp,different-md5,different-timestamp,different-size,deleted,...

The user could specify one or more (comma separated list) and if the file meets any of the criterias, it will be updated (uploaded/deleted) at the destination.

This would clarify the behaviour greatly, especially if you'd mentioned in the documentation that the default behaviour is
--sync-if different-timestamp,different-size.

Use Case

After the first sync command (local->s3), the local files will have an mtime of 0, and the contents in s3 will have a LastModified time of 10 (using relative offsets). When we run the second aws s3 sync command, which is syncing from s3 to local we'll first do the file size check. In this case the file sizes are the same so we look at the last modified time checks. In this case they are different (local == 0, s3 == 10). If we were doing a strict equality comparison then, because the last modified times are different, we would unnecessarily sync the files from s3 to local. So we can say that if the file sizes are the same and the last modified time in s3 is greater (newer) than the local file, then we don't sync. This is the current behavior.

However, this creates a problem if the remote file is updated out of band (via the console or some other SDK) and the size remains the same. If we run aws s3 sync s3://bucket local/ we will not sync the remote file even though we're suppose to.