* Update Gradle in generated Java projects from 2.14.1 to 6.0.1
* Upgrade Java 11 native Gradle from 5.5 to 6.0.1
When Java 11 native client library was introduced it was introduced with an overridden version of the Gradle wrapper (5.5 instead of 2.14.1). From looking at that PR, there doesn't appear to be any specific reason for the versions to differ. This removes the only override of the Gradle wrapper so all the Java projects are up to date and shares the same version.
* implemented core integration with play 2.4 ws
* added shell script to test on CI
* added shell script to composite file for all java generators
* added some comments
changed promise param to Response<T> to allow access to http status code and raw response if needed
* removed unnecessary whitespace changes
* added java7 compatibility, play ws deps to pom.xml
* added generated play24 client
* fixed imports
