fix: Add maven-javadoc-plugin to pom.xml (#2217)

Author: kgala2

core/pom.xml

@@ -238,6 +238,23 @@
           </execution>
         </executions>
       </plugin>
+      <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-javadoc-plugin</artifactId>
+        <version>3.6.3</version> 
+        <configuration>
+          <source>8</source>
+          <sourcepath>${project.build.directory}/generated-sources</sourcepath>
+        </configuration>
+        <executions>
+          <execution>
+            <id>attach-javadocs</id>
+            <goals>
+              <goal>jar</goal>
+            </goals>
+          </execution>
+        </executions>
+      </plugin>
       <plugin>
         <groupId>com.github.os72</groupId>
         <artifactId>protoc-jar-maven-plugin</artifactId>