[FEATURE] Implement Service Discovery configurations on the Gateway CRD

## Summary

As a part of the Service Discovery configurations implementation, the CRD should support the new fields:

```yaml
...
  a2a:
      serviceDiscovery:
      enabled: true
      namespace: 'agents'
      labelSelector: 'inference-gateway.com/a2a-agent=true'
      pollingInterval: '30s'
...
```

When applying this in the Gateway CRD the configmap should be updated with those values.