ditch coverals.io in favor of sonarcloud

[earloc]

replace current code-coverage analysis via coveralls.io in favor of sonar-cloud

• [ ] send coverage to sonarcloud
• [ ] exclude snapshot-files from being counted as LOC
• [ ] update badges in readme.md
• [ ] enable quality-gate check for minimum coverage

[earloc]

Once set up, quality-gate check should be enabled again to require a minimum code-coverage for PRs.