Support multipart/* MediaTypes in RestTemplate

Prior to this commit, RestTemplate posted multipart with Content-Type
"multipart/form-data" even if the FormHttpMessageConverter configured
in the RestTemplate had been configured to support additional multipart
subtypes. This made it impossible to POST form data using a content
type such as "multipart/mixed" or "multipart/related".

This commit addresses this issue by updating FormHttpMessageConverter
to support custom multipart subtypes for writing form data.

For example, the following use case is now supported.

MediaType multipartMixed = new MediaType("multipart", "mixed");

restTemplate.getMessageConverters().stream()
    .filter(FormHttpMessageConverter.class::isInstance)
    .map(FormHttpMessageConverter.class::cast)
    .findFirst()
    .orElseThrow(() ->
        new IllegalStateException("Failed to find FormHttpMessageConverter"))
    .addSupportedMediaTypes(multipartMixed);

MultiValueMap<String, Object> parts = new LinkedMultiValueMap<>();
parts.add("field 1", "value 1");
parts.add("file", new ClassPathResource("myFile.jpg"));

HttpHeaders requestHeaders = new HttpHeaders();
requestHeaders.setContentType(multipartMixed);
HttpEntity<MultiValueMap<String, Object>> requestEntity =
    new HttpEntity<>(parts, requestHeaders);

restTemplate.postForLocation("https://example.com/myFileUpload", requestEntity);

Closes gh-23159

src/docs/asciidoc/integration.adoc

@@ -1142,7 +1142,10 @@ request and response. By default, this converter supports all text media types
 | An `HttpMessageConverter` implementation that can read and write form data from the HTTP
 request and response. By default, this converter reads and writes the
 `application/x-www-form-urlencoded` media type. Form data is read from and written into a
-`MultiValueMap<String, String>`.
+`MultiValueMap<String, String>`. The converter can also write (but not read) multipart
+data read from a `MultiValueMap<String, Object>`. By default, `multipart/form-data` is
+supported. As of Spring Framework 5.2, additional multipart subtypes can be supported for
+writing form data. Consult the javadoc for `FormHttpMessageConverter` for further details.
 
 | `ByteArrayHttpMessageConverter`
 | An `HttpMessageConverter` implementation that can read and write byte arrays from the