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

spring-test/src/test/java/org/springframework/test/web/client/match/MultipartRequestMatchersTests.java

@@ -25,9 +25,10 @@ import org.junit.jupiter.api.Test;
 
 import org.springframework.http.HttpEntity;
 import org.springframework.http.HttpHeaders;
+import org.springframework.http.HttpMethod;
 import org.springframework.http.HttpOutputMessage;
 import org.springframework.http.MediaType;
-import org.springframework.http.converter.FormHttpMessageConverter;
+import org.springframework.http.converter.multipart.MultipartHttpMessageConverter;
 import org.springframework.mock.http.client.MockClientHttpRequest;
 import org.springframework.mock.web.MockMultipartFile;
 import org.springframework.util.LinkedMultiValueMap;
@@ -55,6 +56,7 @@ class MultipartRequestMatchersTests {
 
 	@BeforeEach
 	void setup() {
+		this.request.setMethod(HttpMethod.POST);
 		this.request.getHeaders().setContentType(MediaType.MULTIPART_FORM_DATA);
 	}
 
@@ -188,7 +190,7 @@ class MultipartRequestMatchersTests {
 	}
 
 	private void writeForm() throws IOException {
-		new FormHttpMessageConverter().write(this.input, MediaType.MULTIPART_FORM_DATA,
+		new MultipartHttpMessageConverter().write(this.input, MediaType.MULTIPART_FORM_DATA,
 				new HttpOutputMessage() {
 					@Override
 					public OutputStream getBody() throws IOException {