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