Add support to reference external HandlerMethodArgumentResolver beans which might contain already configured instances (e.g. through a 3rd-party namespace handler).

Users can not mix and match between "inner bean" argument resolver and "external bean" argument resolver. This commit only focuses only on argument-resolver, while the support could be extended to return value handlers as well.

 Issue: SPR-11927

spring-webmvc/src/test/resources/org/springframework/web/servlet/config/mvc-config-argument-resolvers-references.xml

@@ -0,0 +1,17 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<beans xmlns="http://www.springframework.org/schema/beans"
+    xmlns:mvc="http://www.springframework.org/schema/mvc"
+	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+	xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
+		http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd">
+
+	<mvc:annotation-driven>
+		<mvc:argument-resolvers>
+			<bean class="org.springframework.web.servlet.config.TestWebArgumentResolver"/>
+            <ref bean="customArgumentResolver" />
+		</mvc:argument-resolvers>
+	</mvc:annotation-driven>
+
+    <bean id="customArgumentResolver" class="org.springframework.web.servlet.config.TestHandlerMethodArgumentResolver"/>
+
+</beans>