Migrate plugin from maven to gradle

[fscholdei]

Proposed changes

Migrates the build process from maven to gradle.

Your checklist for this pull request

• [x] PR is well described and the description can be used as commit message on squash
• [x] Related issues linked to PR if existing and labels set
• [x] Target branch is not master (in most cases develop should bet the target of choice)
• [x] Code does not conflict with target branch
• [ ] New code is covered with unit tests
• [ ] Changelog entry file created in gradle/changelog
• [ ] Definition of Done's fulfilled: DoD // UI DoD
• [ ] Documentation updated (only necessary for new features or changed behaviour)

Checklist for branch merge request (not required for forks)

• [ ] Branch is green/blue on Jenkins
• [ ] Quality Gate passed on SonarQube