Move multipart support to dedicated converter

Prior to this commit, gh-36255 introduced the new
`MultipartHttpMessageConverter`, focusing on multipart message
conversion in a separate converter. The `FormHttpMessageConverter` did
conflate URL encoded forms and multipart messages in the same converter.

With the introduction of the new converter and related types in the same
package (with `Part`, `FormFieldPart` and `FilePart`), we can now
revisit this arrangement.

This commit restricts the `FormHttpMessageConverter` to URL encoded
forms only and as a result, changes its implementation to only consider
`MultiValueMap<String, String>` types for reading and writing HTTP
messages. Because type erasure, this converter is now a
`SmartHttpMessageConverter` to get better type information with
`ResolvableType`.

As a result, the `AllEncompassingFormHttpMessageConverter` is formally
deprecated and replaced by the `MultipartHttpMessageConverter`, by
setting part converters explicitly in its constructor.

Closes gh-36256

framework-docs/modules/ROOT/pages/integration/rest-clients.adoc

@@ -402,6 +402,27 @@ To serialize only a subset of the object properties, you can specify a {baeldung
 		.toBodilessEntity();
 ----
 
+==== URL encoded Forms
+
+URL encoded forms, using the `"application/x-www-form-urlencoded"` media type, are useful for sending String key/values over the wire.
+This is supported by the `FormHttpMessageConverter`, if the application uses a `MultiValueMap<String, String>` as source instance
+or a target type.
+
+For example:
+
+[source,java,indent=0,subs="verbatim"]
+----
+    MultiValueMap<String, String> form = new LinkedMultiValueMap<>();
+    form.add("project", "Spring Framework");
+    form.add("module", "spring-web");
+    ResponseEntity<Void> response = this.restClient.post()
+            .contentType(MediaType.APPLICATION_FORM_URLENCODED)
+            .body(form)
+            .retrieve()
+            .toBodilessEntity();
+----
+
+
 ==== Multipart
 
 To send multipart data, you need to provide a `MultiValueMap<String, Object>` whose values may be an `Object` for part content, a `Resource` for a file part, or an `HttpEntity` for part content with headers.
@@ -419,18 +440,22 @@ For example:
 	headers.setContentType(MediaType.APPLICATION_XML);
 	parts.add("xmlPart", new HttpEntity<>(myBean, headers));
 	
-	// send using RestClient.post or RestTemplate.postForEntity
+    ResponseEntity<Void> response = this.restClient.post()
+            .contentType(MediaType.MULTIPART_FORM_DATA)
+            .body(parts)
+            .retrieve()
+            .toBodilessEntity();
 ----
 
 In most cases, you do not have to specify the `Content-Type` for each part.
 The content type is determined automatically based on the `HttpMessageConverter` chosen to serialize it or, in the case of a `Resource`, based on the file extension.
 If necessary, you can explicitly provide the `MediaType` with an `HttpEntity` wrapper.
 
-Once the `MultiValueMap` is ready, you can use it as the body of a `POST` request, using `RestClient.post().body(parts)` (or `RestTemplate.postForObject`).
+The `Content-Type` is set to `multipart/form-data` by the `MultiPartHttpMessageConverter`.
+As seen in the previous section, `MultiValueMap` types can also be used for URL encoded forms.
+It is preferable to explicitly set the media type in the `Content-Type` or `Accept` HTTP request headers to ensure that the expected
+message converter is used.
 
-If the `MultiValueMap` contains at least one non-`String` value, the `Content-Type` is set to `multipart/form-data` by the `FormHttpMessageConverter`.
-If the `MultiValueMap` has `String` values, the `Content-Type` defaults to `application/x-www-form-urlencoded`.
-If necessary the `Content-Type` may also be set explicitly.
 
 [[rest-request-factories]]
 === Client Request Factories

framework-docs/modules/ROOT/pages/web/webmvc/message-converters.adoc

@@ -23,13 +23,16 @@ For all converters, a default media type is used, but you can override it by set
 By default, this converter supports all text media types(`text/{asterisk}`) and writes with a `Content-Type` of `text/plain`.
 
 | `FormHttpMessageConverter`
-| An `HttpMessageConverter` implementation that can read and write form data from the HTTP request and response.
+| An `HttpMessageConverter` implementation that can read and write URL encoded forms.
 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>`.
-The converter can also write (but not read) multipart data read from a `MultiValueMap<String, Object>`.
-By default, `multipart/form-data` is supported.
-Additional multipart subtypes can be supported for writing form data.
-Consult the javadoc for `FormHttpMessageConverter` for further details.
+
+| `MultipartHttpMessageConverter`
+| An `HttpMessageConverter` implementation that can read and write multipart messages.
+`MultiValueMap<String, Object>` can be written to multipart messages, converting each part independently using
+the configured message converters. Multipart messages can be read into `MultiValueMap<String, Part>`, each value
+being a `Part` or one of its subtypes (`FormFieldPart` and `FilePart`).
+By default, `multipart/form-data` is supported. Additional multipart subtypes can be supported for writing form data.
 
 | `ByteArrayHttpMessageConverter`
 | An `HttpMessageConverter` implementation that can read and write byte arrays from the HTTP request and response.