add support to gradle

[pcariel]

So many repositories in networknt are using Gradle with Kotlin DSL. It should be easy to add that so that we can either let the user to choose one in the light-codegen config.json or just generate both so that user can invoke one of them in the command line. I don't know if there are any conflict between Maven and Gradle though.

[stevehu]

@pcariel We have added a new generator for OpenAPI Kotlin and Gradle is the default build system. Will soon add Gradle to other generators so that users can choose between Maven and Gradle. Also, as you can see we have added both Maven and Gradle wrapper so that it is not necessary to install Maven and Gradle separately for users. Stay tuned...