Align HttpMessageConverters builder with WebFlux codecs variants

[bclozel]

As of #33894, we introduced a new HttpMessageConverters API. While this achieved our goal of focusing converters classpath detection in a single place and avoiding waste, a single HttpMessageConverters instance for both client and server added more complexity for developers.

We should better align the API here with the WebFlux CodecsConfigurer to opt for a client/server flavor as the first step in the builder. While this make the sharing of converter instances between server and client, this makes a simpler API and separates concerns.

[bclozel]

@mp911de I have updated the API as discussed previously. This shouldn't change much for you.

You will probably have to change a WebMvcConfigurer method signature from public void configureMessageConverters(HttpMessageConverters.Builder builder) to public void configureMessageConverters(HttpMessageConverters.ServerBuilder builder). The builder API itself should not change here.

Let me know if I can help.