[WIP] Re-Instating SonarCloud (fixes #81) (#373)

* Introducing Jacoco Tetst Report Generation

* Running Jacoco Report and Guarding SonarCloud for External PRs

* Add coverage badge

Co-authored-by: Hee Tatt Ooi <hee.tatt.ooi@sap.com>
Co-authored-by: Jakob Möller <jakob.moeller@sap.com>