Allow choice between Forwarded and X-Forwarded headers

This commit introduces a constructor argument to select whether
to use the standard "Forwarded" header or the "X-Forwarded-*"
alternative headers. A separate property to control support for
X-Forwarded-Prefix.

Closes gh-37090

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/RequestMappingIntegrationTests.java

@@ -148,7 +148,7 @@ class RequestMappingIntegrationTests extends AbstractRequestMappingIntegrationTe
 
 		@Bean
 		public ForwardedHeaderTransformer forwardedHeaderTransformer() {
-			return new ForwardedHeaderTransformer();
+			return new ForwardedHeaderTransformer(true);
 		}
 	}