test: establish integration tests

[goatfryed]

We don't just want to manually review the generated k8s manifests and optionally manually apply them to a test cluster, but we want to automate this.

We want to install a sample application with the chart on a test cluster and verify kubernetes setup and application properties.
Mainly

• health checks
• mounting external configmaps & secrets
• application properties

goatfryed/spring-boot-diagnostic (github/dockerhub) has been prepared to validate

• Define integration test concept
• CICD Integration Test setup
  • consider k3d cluster for independent test in the github runner
• Select test framework
  • investigate go terratest