feat: support per-route container image override

[dcruver]

Summary

Adds an optional spec.image field to the IntegrationRoute CRD, allowing individual routes to specify their own container image instead of using the cluster-wide default.

Motivation

Currently, all IntegrationRoutes in a keip cluster share the same container image defined in keip-controller-props. This means if you have multiple applications with different classpaths (e.g., different custom beans or dependencies), you must either:

1. Bundle everything into a single fat image
2. Run separate keip clusters

Neither is ideal. Per-route images let different applications coexist naturally in the same cluster.

Changes

• CRD (operator/crd/crd.yaml): Added spec.image (optional string) to v1alpha2 schema
• Webhook (webapp/core/sync.py): Resolve image from spec.image with fallback to global INTEGRATION_CONTAINER_IMAGE
• Tests (webapp/core/test/test_sync.py): Two new tests verifying override and default fallback

Usage

apiVersion: keip.codice.org/v1alpha2
kind: IntegrationRoute
metadata:
  name: my-custom-route
spec:
  image: "registry.example.com/my-app:1.0"
  routeConfigMap: my-route-xml
  replicas: 1

When spec.image is omitted, behavior is unchanged — the global default from the controller ConfigMap is used.

Backward Compatibility

Fully backward compatible. The field is optional with no default in the CRD schema. Existing IntegrationRoutes without spec.image continue to work exactly as before.