Moving project to Gradle

There is an idea to move the project to Gradle, for two reasons:
1) Maven builds take too long to complete. Gradle's incremental builds are much more powerful in recent years at cutting off the unnecessary work.
2) We can run IntelliJ inspections locally using a plugin: https://github.com/JetBrains/inspection-plugin

This message was originally posted here: https://github.com/apache/incubator-druid/issues/7834#issuecomment-500963555

Note: I don't plan to work on this in the near future.

Start by reviewing the original discussion in apache/druid#7834 and the proposal in this issue, then examine how the current Maven build is organized. The work is done when the project builds with Gradle, preserves the existing build behavior, improves incremental build time, and supports local IntelliJ inspections through the referenced plugin.

Written by the indexing model from the issue text.