feat: update trustDomain visibility in Istio custom resource details in Busola

Author: kubicar

config/ui-extensions/istios/details

@@ -26,7 +26,7 @@ body:
       placeholder: '1'
     - source: spec.config.trustDomain
       name: config.trustDomain
-      placeholder: 'cluster.local'
+      visibility: '$exists($value)'
     - source: spec.compatibilityMode
       name: compatibilityMode
       placeholder: 'false'