Support managed network creation in StackitCluster #9

Description

Currently, StackitCluster requires users to provide an existing network via spec.network.id. Bring-your-own (BYO) network through .spec.network.id should become optional and instead we want to introduce automated network lifecycle management (managed network creation and deletion) when omitted.

Specification

1. API:
   • Make .spec.network.id optional in StackitCluster
2. Reconciliation (Creation):
   • Omitted spec.network.id (Managed / Default): Automatically provision a dedicated STACKIT network for the cluster, tag it as managed (owned), and store the created ID
   • Provided spec.network.id (BYO / Existing behavior): Use the specified preexisting network as unmanaged (shared), preserving current behavior.
3. Reconciliation (Deletion):
   • BYO: Continue skipping network/subnet deletion when spec.network.id was user-provided.
   • Managed: Attempt deletion of the created network. If out-of-band dependencies or active interfaces remain, fail the step, set a condition/event on StackitCluster, and requeue until the network is clear to delete.

This aligns directly with standard behaviour across other kubernetes-sigs CAPI providers (such as CAPA, CAPZ, and CAPG).