Allow platform Kubernetes monitoring to reference an existing Observability instance #88

The root management module and each platform Kubernetes module can create separate STACKIT Observability instances. Users need to choose a compatible existing or centrally declared instance for SKE monitoring instead of necessarily creating another.

Current behavior: src/modules/platform-kubernetes/2-observability.tf creates an instance whenever observability.enabled is true; 3-cluster.tf directly references that resource. The root observability input creates a separate instance in the management project. Management currently exposes no Observability instance output for this reference.

Proposed scope:

• Explicit monitoring modes disabled / create managed instance / reference compatible instance.
• Publish needed instance ID and endpoint outputs and wire declarative references without storing credentials in configuration.
• Verify SKE project/region/permissions requirements for cross-project references before offering central management-instance selection.
• Preserve current resource addresses/defaults; prevent silent destruction when changing ownership to a reference.
• Validate dangling/disabled references and test reuse creates no duplicate instance.
• Keep cluster-system monitoring separate from application telemetry and Telemetry Router.

Official SKE documentation describes choosing an existing instance: https://docs.stackit.cloud/products/runtime/kubernetes-engine/how-tos/monitor-your-clusters/