Add support for Save-Data request header #1258

https://httpwg.github.io/http-extensions/client-hints.html#the-save-data-hint

When the Save-Data header is present, PageSpeed should automatically adjust its filters and optimizations to reduce the amount of served bytes -- e.g. compress images with lower quality. Ideally, this should be an "on by default" behavior, but one that can be (re)configured by the site owner.