Transform the Gradle script files into a Gradle plugin.

[rkrisztian]

The Gradle plugin way comes with the following benefits:

• Vulnerabilty checks for dependencies.
• Ability to write automated tests and run them in a CI build. (I have started it but we should add more later.)
• Possibility to use lazy properties in the future. (Should help with eliminating the leftover reliance on 'afterEvaluate` and strict ordering of configurations.)

I have used more idiomatic Gradle/Groovy code style in some places, as well as the Task Configuration Avoidance API.

I have left the original code in, in case they are still needed for compatibility with previous releases.

[rkrisztian]

I did not add any publication tasks. But it shouldn't be hard:
https://docs.gradle.org/current/userguide/publishing_gradle_plugins.html

[rkrisztian]

One more thing to add is that now the JSONs are served as resources in the jar. If this kind of releasing is not ideal, then we should discuss a different approach. But I still would not reinvent the wheel and use a HTTP client library to download the JSONs, but configure some Ivy Repository pattern.

I also eliminated the use of /tmp, because it's an unusual pattern for Gradle builds. I'd rather have dependencies coming from more common places like ~/.m2, or ~/.gradle, and put all intermediary files inside the Gradle project's build directory.