Added readinessProbe to webhook deployment

[sotoiwa]

This PR adds a readinessProbe to prevent traffic from being routed to webhook pods before they're ready to handle requests.

Problem

Without readinessProbe, webhook pods receive traffic immediately after startup, which can cause "connection refused" errors during cluster operations (upgrades, rolling updates). Due to failurePolicy: Ignore, pods are created without expected IAM role mutations.

Solution

Added readinessProbe using the existing /healthz endpoint:

        livenessProbe:
          httpGet:
            path: /healthz
            port: 443
            scheme: HTTPS
          initialDelaySeconds: 10
          periodSeconds: 10
          failureThreshold: 3
        readinessProbe:
          httpGet:
            path: /healthz
            port: 443
            scheme: HTTPS
          initialDelaySeconds: 5
          periodSeconds: 5
          failureThreshold: 2

Related Issues

• Fixes Specify liveness & readiness probes for the pod-identity-webhook  #111
• Related to Add /readyz and a useful /healthz #98 (future enhancement for comprehensive health checks)

While the current /healthz endpoint is basic, it ensures the webhook server is at least accepting connections before receiving traffic, significantly improving reliability during cluster operations.

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