Allow protocol relative URLs in CssLink Transformer

This commit allows the use of "protcol relative URLs" (i.e. URLs without
scheme, starting with `//`), often used to serve resources automatically
from https or http with third party domains.

This syntax is allowed by RFC 3986.

Issue: SPR-12632

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

@@ -1,2 +1,3 @@
 @import url("http://example.org/fonts/css");
-body { background: url("file:///home/spring/image.png") }
+body { background: url("file:///home/spring/image.png") }
+figure { background: url("//example.org/style.css")}