[Java] Templates to support google-api-client library (#6838)

* Add support for the google-api-client

* When there's no return type, just return (no parsing the response)

* Update the deserialization to use a Jackson typeref.

* Delete the auth mustache files as we don't need them

* Stop generating unneeded auth files; get SBT/gradle working with the generated module

* Add builder-style methods to the google-api-client ApiClient for getting API instances

* Update the README to reflect the new client library option

* Generated overloaded methods to send query params as a Map<String, Object>

* Add files for Sample codes

* Add type in angle brackets to support Java 6

* Required query params must be added to the Map of params. Update sample code.

* Use explicit types instead of diamonds to support Java6 in one more place.

* Clean up javadoc warnings in generated code / remove pointless return statements

* Clean up extra newlines in generated code

bin/java-petstore-google-api-client.sh

@@ -0,0 +1,34 @@
+#!/bin/sh
+
+SCRIPT="$0"
+
+while [ -h "$SCRIPT" ] ; do
+  ls=`ls -ld "$SCRIPT"`
+  link=`expr "$ls" : '.*-> \(.*\)$'`
+  if expr "$link" : '/.*' > /dev/null; then
+    SCRIPT="$link"
+  else
+    SCRIPT=`dirname "$SCRIPT"`/"$link"
+  fi
+done
+
+if [ ! -d "${APP_DIR}" ]; then
+  APP_DIR=`dirname "$SCRIPT"`/..
+  APP_DIR=`cd "${APP_DIR}"; pwd`
+fi
+
+executable="./modules/swagger-codegen-cli/target/swagger-codegen-cli.jar"
+
+if [ ! -f "$executable" ]
+then
+  mvn clean package
+fi
+
+# if you've executed sbt assembly previously it will use that instead.
+export JAVA_OPTS="${JAVA_OPTS} -XX:MaxPermSize=256M -Xmx1024M -DloggerPath=conf/log4j.properties"
+ags="$@ generate -t modules/swagger-codegen/src/main/resources/Java/libraries/google-api-client -i modules/swagger-codegen/src/test/resources/2_0/petstore-with-fake-endpoints-models-for-testing.yaml -l java -c bin/java-petstore-google-api-client.json -o samples/client/petstore/java/google-api-client -DhideGenerationTimestamp=true"
+
+echo "Removing files and folders under samples/client/petstore/java/google-api-client/src/main"
+rm -rf samples/client/petstore/java/google-api-client/src/main
+find samples/client/petstore/java/google-api-client -maxdepth 1 -type f ! -name "README.md" -exec rm {} +
+java $JAVA_OPTS -jar $executable $ags