[Java] add API helper function to Java and Add licenseInfo to all Java lib template

[shijinkui]

PR checklist

• Read the contribution guidelines.
• Ran the shell script below to update Petstore sample

    ./bin/scala-petstore.sh
    ./bin/akka-scala-petstore.sh
    ./bin/scala-lagom-server-petstore.sh
    ./bin/scalatra-petstore-server.sh
    ./bin/scalaz-petstore.sh
    ./bin/akka-scala-petstore.sh
    ./bin/java-play-framework-petstore-server-all.sh
    ./bin/android-petstore-all.sh
    ./bin/groovy-petstore.sh
    ./bin/kotlin-client-petstore.sh
  

• issue [Java] add API helper function to Java and Add licenseInfo to all Java lib template #7414

Description of the PR

1. delete blank line between license and package line
2. add {{>licenseInfo}} to all class file
3. generate java-petstore
4. add function setApiEndpoint(String apiEndpoint) in java/api.mustache
   like: private final ClusterApi api = new ClusterApi().setApiEndpoint("https://xxx.com");

cc @bbdouglas @JFCote @sreeshas @jfiala @lukoyanov @cbornet

[wing328]

add function setApiEndpoint(String apiEndpoint) in java/api.mustache
like: private final ClusterApi api = new ClusterApi().setApiEndpoint("https://xxx.com");

Please do this in a separate PR.

I believe it's currently doable with the Configuration class without introducing the setApiEndpoint method.