Support programmatic multipart/form-data responses

With these changes, users can also manually append the parts of the form using the class `MultipartFormDataOutput` as:

```java
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;

import org.jboss.resteasy.reactive.server.core.multipart.MultipartFormDataOutput;

@path("multipart")
public class Endpoint {

    @get
    @produces(MediaType.MULTIPART_FORM_DATA)
    @path("file")
    public MultipartFormDataOutput getFile() {
        MultipartFormDataOutput form = new MultipartFormDataOutput();
        form.addFormData("person", new Person("John"), MediaType.APPLICATION_JSON_TYPE);
        form.addFormData("status", "a status", MediaType.TEXT_PLAIN_TYPE)
                .getHeaders().putSingle("extra-header", "extra-value");
        return form;
    }
}
```

Fix quarkusio#28631

This last approach allows you adding extra headers to the output part.

(cherry picked from commit 6967fbc)

docs/src/main/asciidoc/resteasy-reactive.adoc

@@ -544,6 +544,35 @@ public class Endpoint {
 }
 ----
 
+Additionally, you can also manually append the parts of the form using the class `MultipartFormDataOutput` as:
+
+[source,java]
+----
+import javax.ws.rs.GET;
+import javax.ws.rs.Path;
+import javax.ws.rs.Produces;
+import javax.ws.rs.core.MediaType;
+
+import org.jboss.resteasy.reactive.server.core.multipart.MultipartFormDataOutput;
+
+@Path("multipart")
+public class Endpoint {
+
+    @GET
+    @Produces(MediaType.MULTIPART_FORM_DATA)
+    @Path("file")
+    public MultipartFormDataOutput getFile() {
+        MultipartFormDataOutput form = new MultipartFormDataOutput();
+        form.addFormData("person", new Person("John"), MediaType.APPLICATION_JSON_TYPE);
+        form.addFormData("status", "a status", MediaType.TEXT_PLAIN_TYPE)
+                .getHeaders().putSingle("extra-header", "extra-value");
+        return form;
+    }
+}
+----
+
+This last approach allows you adding extra headers to the output part. 
+
 WARNING: For the time being, returning Multipart data is limited to be blocking endpoints.
 
 ==== Handling malformed input

extensions/resteasy-reactive/quarkus-resteasy-reactive-jsonb/deployment/src/test/java/io/quarkus/resteasy/reactive/jsonb/deployment/test/MultipartOutputTest.java

@@ -38,7 +38,6 @@ public void testSimple() {
                 .then()
                 .contentType(ContentType.MULTIPART)
                 .statusCode(200)
-                .log().all()
                 .extract().asString();
 
         assertContains(response, "name", MediaType.TEXT_PLAIN, EXPECTED_RESPONSE_NAME);

extensions/resteasy-reactive/quarkus-resteasy-reactive/deployment/src/main/java/io/quarkus/resteasy/reactive/server/deployment/QuarkusMultipartReturnTypeHandler.java

@@ -11,6 +11,7 @@
 import org.jboss.resteasy.reactive.common.processor.AdditionalWriters;
 import org.jboss.resteasy.reactive.common.processor.EndpointIndexer;
 import org.jboss.resteasy.reactive.server.core.multipart.MultipartMessageBodyWriter;
+import org.jboss.resteasy.reactive.server.processor.generation.multipart.FormDataOutputMapperGenerator;
 
 import io.quarkus.deployment.GeneratedClassGizmoAdaptor;
 import io.quarkus.deployment.annotations.BuildProducer;

extensions/resteasy-reactive/quarkus-resteasy-reactive/deployment/src/test/java/io/quarkus/resteasy/reactive/server/test/multipart/MultipartOutputResource.java

@@ -11,6 +11,7 @@
 import javax.ws.rs.core.MediaType;
 
 import org.jboss.resteasy.reactive.RestResponse;
+import org.jboss.resteasy.reactive.server.core.multipart.MultipartFormDataOutput;
 
 @Path("/multipart/output")
 public class MultipartOutputResource {
@@ -51,6 +52,20 @@ public RestResponse<MultipartOutputResponse> restResponse() {
         return RestResponse.ResponseBuilder.ok(response).header("foo", "bar").build();
     }
 
+    @GET
+    @Path("/with-form-data")
+    @Produces(MediaType.MULTIPART_FORM_DATA)
+    public RestResponse<MultipartFormDataOutput> withFormDataOutput() {
+        MultipartFormDataOutput form = new MultipartFormDataOutput();
+        form.addFormData("name", RESPONSE_NAME, MediaType.TEXT_PLAIN_TYPE);
+        form.addFormData("custom-surname", RESPONSE_SURNAME, MediaType.TEXT_PLAIN_TYPE);
+        form.addFormData("custom-status", RESPONSE_STATUS, MediaType.TEXT_PLAIN_TYPE)
+                .getHeaders().putSingle("extra-header", "extra-value");
+        form.addFormData("values", RESPONSE_VALUES, MediaType.TEXT_PLAIN_TYPE);
+        form.addFormData("active", RESPONSE_ACTIVE, MediaType.TEXT_PLAIN_TYPE);
+        return RestResponse.ResponseBuilder.ok(form).header("foo", "bar").build();
+    }
+
     @GET
     @Path("/string")
     @Produces(MediaType.MULTIPART_FORM_DATA)

extensions/resteasy-reactive/quarkus-resteasy-reactive/deployment/src/test/java/io/quarkus/resteasy/reactive/server/test/multipart/MultipartOutputUsingBlockingEndpointsTest.java

@@ -63,6 +63,22 @@ public void testRestResponse() {
         assertContainsValue(response, "num", MediaType.TEXT_PLAIN, "0");
     }
 
+    @Test
+    public void testWithFormData() {
+        String response = RestAssured.get("/multipart/output/with-form-data")
+                .then()
+                .log().all()
+                .contentType(ContentType.MULTIPART)
+                .statusCode(200)
+                .extract().asString();
+
+        assertContainsValue(response, "name", MediaType.TEXT_PLAIN, MultipartOutputResource.RESPONSE_NAME);
+        assertContainsValue(response, "custom-surname", MediaType.TEXT_PLAIN, MultipartOutputResource.RESPONSE_SURNAME);
+        assertContainsValue(response, "custom-status", MediaType.TEXT_PLAIN, MultipartOutputResource.RESPONSE_STATUS);
+        assertContainsValue(response, "active", MediaType.TEXT_PLAIN, MultipartOutputResource.RESPONSE_ACTIVE);
+        assertContainsValue(response, "values", MediaType.TEXT_PLAIN, "[one, two]");
+    }
+
     @Test
     public void testString() {
         RestAssured.get("/multipart/output/string")