Move multipart support out of FormHttpMessageConverter

[bclozel]

With the introduction of the new multipart converter in #36255, we need to clarify the responsibilities between MultipartHttpMessageConverter and the existing FormHttpMessageConverter.

FormHttpMessageConverter should focus on "application/x-www-form-urlencoded" media types only and deal with MultiValueMap<String, Object> payloads. MultipartHttpMessageConverter should focus on "multipart/*" media types only and deal with MultiValueMap<String, Part> payloads for reading and MultiValueMap<String, Object> for writing.

We should also deprecate AllEncompassingFormHttpMessageConverter completely, as instantiating a MultipartHttpMessageConverter with part converters should achieve the same and avoid duplication.

This will require significant changes in HttpMessageConverters and our existing test suite to keep existing features in place.