feat: Add JavadocJar Task to build.gradle for self service libraries (#2593)

Fixes #1367

Author: lqiu96

rules_java_gapic/resources/gradle/client_grpc.gradle.tmpl

@@ -37,6 +37,15 @@ clean {
   delete 'all-jars'
 }
 
+task javadocJar(type: Jar) {
+  archiveClassifier = 'javadoc'
+  from javadoc
+}
+
+artifacts {
+  archives javadocJar
+}
+
 task allJars(type: Copy) {
   dependsOn test, jar
   into 'all-jars'

rules_java_gapic/resources/gradle/client_grpcrest.gradle.tmpl

@@ -39,6 +39,15 @@ clean {
   delete 'all-jars'
 }
 
+task javadocJar(type: Jar) {
+  archiveClassifier = 'javadoc'
+  from javadoc
+}
+
+artifacts {
+  archives javadocJar
+}
+
 task allJars(type: Copy) {
   dependsOn test, jar
   into 'all-jars'

rules_java_gapic/resources/gradle/client_rest.gradle.tmpl

@@ -34,6 +34,15 @@ clean {
   delete 'all-jars'
 }
 
+task javadocJar(type: Jar) {
+  archiveClassifier = 'javadoc'
+  from javadoc
+}
+
+artifacts {
+  archives javadocJar
+}
+
 task allJars(type: Copy) {
   dependsOn test, jar
   into 'all-jars'