Custom Button in Editor Window

There are buttons at the left of java editor tabs - run/debug main method or unit tests - like a green arrow icon.

I would like to have something like that to run some of methods with special annotations. Let's say to create a custom test runner.
As I understand it is about to create an IntelliJ Platform Plugins.

While I am learning available docs could you please refer me to pieces of code implementing this buttons ? I saw junit plugin from intellij-community source, but cannot realise details how exactly such buttons were added.