GH-39327: [Java] define assemble descriptor for new custom maven plug…

…in project (#39331)

### Rationale for this change

To closes #39327

### What changes are included in this PR?

GitHub CI validation needs to [run](https://github.com/apache/arrow/blob/main/ci/scripts/java_full_build.sh#L52) `assembly:single` for that reason is needed to setup a descriptor ref. In the case of this maven plugin, I only propose to include "src" as part of the resources.

### Are these changes tested?

Yes, by 
````
mvn clean \
    install \
    assembly:single \
    source:jar \
    javadoc:jar \
    -Papache-release \
    -DdescriptorId=source-release
````

### Are there any user-facing changes?

No.

* Closes: #39327

Lead-authored-by: david dali susanibar arce <[email protected]>
Co-authored-by: Sutou Kouhei <[email protected]>
Signed-off-by: David Li <[email protected]>

java/maven/pom.xml

@@ -281,6 +281,27 @@
           </execution>
         </executions>
       </plugin>
+      <!--
+      Generate custom Arrow Java Maven Plugin for module-info.java compiler.
+      Needed to pass GitHub CI validation `$ mvn assembly:single`
+      -->
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-assembly-plugin</artifactId>
+        <executions>
+          <execution>
+            <phase>package</phase>
+            <goals>
+              <goal>single</goal>
+            </goals>
+          </execution>
+        </executions>
+        <configuration>
+          <descriptorRefs>
+            <descriptorRef>src</descriptorRef>
+          </descriptorRefs>
+        </configuration>
+      </plugin>
     </plugins>
   </build>