[cleanup] Update build process and project structure

[Dariusz Kuc]

Currently project is using Maven to build libraries and the example app. While Maven is great for simple setups, it is also severely limiting whenever there is a need for some custom setup (e.g. there is no need to publish parent pom.xml as it is not useful for the clients, there is also no need to publish example app either....).

By moving to Gradle we could utilize composite build to isolate examples from the main library codebase.

Proposed new project structure

|- graphql-java-support
|- graphql-java-support-api
|- examples
  |- spring-example (based on the new `spring-graphql` project)
  |- dgs-example
  |- java-kickstart-example (split from current example app)
  |- java-tools-example (split from current example app)

[Dariusz Kuc]

Build was migrated to Gradle in #177.

Project structure refactoring is TODO.

[Dariusz Kuc]

Not needed. Example integrations are now in separate repositories and linked from project README.