Java: Add source plugin (#1925)

This ensures that a `mvn install` will also post source JARs into the
local repository, helping developers navigate through the source code
while debugging.

java/pom.xml

@@ -1,7 +1,8 @@
 <?xml version="1.0" encoding="UTF-8"?>
 
-<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
 
     <modelVersion>4.0.0</modelVersion>
 
@@ -441,6 +442,20 @@
                     </execution>
                 </executions>
             </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-source-plugin</artifactId>
+                <version>3.3.0</version>
+                <executions>
+                    <execution>
+                        <id>attach-sources</id>
+                        <phase>verify</phase>
+                        <goals>
+                            <goal>jar-no-fork</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
         </plugins>
 
     </build>
@@ -668,7 +683,7 @@
                                 <licenseHeader>
                                     <content>// Copyright (c) Microsoft. All rights reserved.</content>
                                 </licenseHeader>
-                                <toggleOffOn/>
+                                <toggleOffOn />
                             </java>
                         </configuration>
                     </plugin>