Make it easier to provide a custom RequestToViewNameTranslator

[wilkinsona]

@dsyer wants to customize the RequestToViewNameTranslator. It can be done today with a BeanPostProcessor for the viewNameTranslator bean. We could make it easier by overriding the viewNameTranslator() method and making it @ConditionalOnMissingBean(name = DispatcherServlet.REQUEST_TO_VIEW_NAME_TRANSLATOR_BEAN_NAME). This would be similar to what we did with flashMapManager() in #24207. Interestingly, we considered doing the same for RequestToViewNameTranslator at that time but @rstoyanchev suggested that we leave it out for now. Looks like the time has come.

[philwebb]

Closing in favor of PR #40874. Thanks @rohitp-a