Support per-cluster Incus client configuration

[tsuzu]

Summary

• Add support for configuring Incus client per cluster resource
• Replace single global Incus client with per-cluster client factory
• Enable multi-cluster/multi-project support

Changes

• New ClientFactory: Creates Incus clients based on cluster configuration
• Updated Controller: Uses ClientFactory to get appropriate client for each cluster
• Per-cluster Config: Each IncusCluster can specify its own endpoint, project, and credentials
• Secret Integration: Reads TLS certificates and connection options from Kubernetes secrets

Technical Details

• pkg/incus/client_factory.go: New factory interface and implementation
• IncusMachineReconciler: Updated to use factory instead of single client
• main.go: Removed global Incus connection setup
• Secret format supports: tls-ca, tls-client-cert, tls-client-key, insecure-skip-verify

Test Plan

• Code compiles successfully
• CI tests pass
• Integration tests with multiple Incus servers
• Verify secret-based authentication works

This enables the provider to manage machines across multiple Incus servers and projects simultaneously.