Assignments supported by gradle

[palves-ulht]

Right now, Drop Project requires all assignments to be maven projects, since it relies on maven invoker (a plugin) to execute the build pipeline.

It shouldn't be too hard to also support assignments based on gradle, using the gradle tooling api (https://docs.gradle.org/current/userguide/third_party_integration.html#embedding). This would open the door for automated assessment of android projects, for example.