Introduce LocaleContextResolver in WebFlux

This commit introduces LocaleContextResolver interface, which is used
at ServerWebExchange level to resolve Locale, TimeZone and other i18n
related informations.

It follows Spring MVC locale resolution patterns with a few differences:
 - Only LocaleContextResolver is supported since LocaleResolver is less
   flexible
 - Support is implemented in the org.springframework.web.server.i18n
   package of spring-web module rather than in spring-webflux in order
   to be able to leverage it at ServerWebExchange level

2 implementations are provided:
 - FixedLocaleContextResolver
 - AcceptHeaderLocaleContextResolver

It can be configured with both functional or annotation-based APIs.

Issue: SPR-15036

spring-test/src/main/java/org/springframework/mock/http/server/reactive/MockServerWebExchange.java

@@ -18,6 +18,7 @@ package org.springframework.mock.http.server.reactive;
 import org.springframework.http.codec.ServerCodecConfigurer;
 import org.springframework.web.server.ServerWebExchangeDecorator;
 import org.springframework.web.server.adapter.DefaultServerWebExchange;
+import org.springframework.web.server.i18n.AcceptHeaderLocaleContextResolver;
 import org.springframework.web.server.session.DefaultWebSessionManager;
 
 /**
@@ -37,7 +38,7 @@ public class MockServerWebExchange extends ServerWebExchangeDecorator {
 	public MockServerWebExchange(MockServerHttpRequest request) {
 		super(new DefaultServerWebExchange(
 				request, new MockServerHttpResponse(), new DefaultWebSessionManager(),
-				ServerCodecConfigurer.create()));
+				ServerCodecConfigurer.create(), new AcceptHeaderLocaleContextResolver()));
 	}