feat(cozystack-api): Implement hot reload for ApplicationDefinition changes

[kvaps]

Summary

Currently, when ApplicationDefinition resources change, cozystack-controller triggers a full restart of the cozystack-api pod by patching the pod template annotation. This causes:

1. Race condition at startup — if CRD doesn't exist yet, apiserver waits up to 30 minutes
2. Downtime — full process restart on every CRD change
3. External dependency — relies on controller to manage apiserver lifecycle

Proposal

Implement hot reload inside apiserver following the pattern from apiextensions-apiserver:

• apiserver watches ApplicationDefinition directly via controller-runtime informer
• On changes, atomically updates REST handlers using atomic.Value
• Updates OpenAPI schema dynamically via OpenAPIVersionedService.UpdateSpec()
• No process restart required

Implementation outline

1. Create DynamicResourceHandler with atomic storage for REST handlers
2. Create HotReloadController that watches CRD and computes diffs
3. Remove blocking retry loop in Complete() — start with empty config
4. Remove restart logic from cozystackresource_controller.go

Benefits

• Zero downtime on CRD changes
• Faster startup (no waiting for CRD)
• Simpler architecture (no external restart trigger)
• Better resilience