Apply codecs auto-configuration to webflux server and webclient

[bclozel]

For Spring MVC, the HttpMessageConvertersAutoConfiguration configures the appropriate message converters for the server and the client (RestTemplate).

We should do the same for WebFlux, using ServerCodecConfigurer and ClientCodecConfigurer.

[snicoll]

Interesting convo in #9485

[alimate]

Probably the same issue

[bclozel]

Some changes are required in Framework to consistently apply codec configuration on both client and server sides, see SPR-15682.