Custom export buildscript support

[alexstaeding]

The Gradle projects exported by the GradleSubmissionExporter currently use a hardcoded Gradle configuration defined by the following build.gradle.kts_.

This hardcoded configuration works most of the time. However, there are cases (such as custom dependencies), where it is necessary to modify the exported Gradle configuration. This should be supported by the API.

The missing functionality would most likely be appropriate in RubricConfiguration.