Create a Java user demo

[cristiklein]

Write code equivalent to:

• https://github.com/elastisys/compliantkubernetes/tree/main/user-demo
• https://github.com/elastisys/compliantkubernetes/tree/main/user-demo-dotnet

... but in Java.

MUST HAVEs:

• Exposes the /, /users, and /crash REST endpoints.
• Uses structured logging via stdout.
• Exposes a Prometheus-compliant /metrics endpoint.

NICE TO HAVEs:

• Existing scripts just work
• Existing Helm Chart just works
• Existing Grafana dashboard just works

Implementation hints:

• Spring boot