Add ResourceTransformer and CSS link implementation

This change adds a ResourceTransformer that can be invoked in a chain
after resource resolution. The CssLinkResourceTransformer modifies a
CSS file being served in order to update its @import and url() links
(e.g. to images or other CSS files) to match the resource resolution
strategy (e.g. adding MD5 content-based hashes).

Issue: SPR-11800