Use JavaExec instead of DefaultTask for Gradle Task grader{Private,Public}Run

[dstngl]

Currently, class GraderRunTask is using DefaultTask as basic class.

In IntelliJ, when a DefaultTask is started, gradle uses resources from the IDE. Especially, gradle does not create a new process. It is very likely that this behavior results in the following issues:

• On the one side, the process running the gradle task terminates only when the IDE is terminates. This might be the reason for overflowing heap space which is an known issue.
• On the other side, the process of the IDE does not allow to create java agents at runtime. This can be explicitly allowed for JavaExec gradle tasks.

Using JavaExec instead of DefaultTask might solve these issues.