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

spring-webmvc/src/test/resources/org/springframework/web/servlet/resource/test/foo.css.less

@@ -1 +0,0 @@
-h1 { color:red; }

spring-webmvc/src/test/resources/org/springframework/web/servlet/resource/test/main.css

@@ -0,0 +1,11 @@
+
+@import url("bar.css");
+@import url('bar.css');
+@import url(bar.css);
+
+@import "foo.css";
+@import 'foo.css';
+
+body { background: url("images/image.png") }
+
+li { list-style: url(http://www.example.com/redball.png) disc }

spring-webmvc/src/test/resources/org/springframework/web/servlet/resource/test/zoo.css.less

@@ -1 +0,0 @@
-h1 { color:red; }