Support for ability to select zone in Managed Vertex AI Notebooks/Workbench

Thanks for stopping by to let us know something could be better!

**PLEASE READ**: If you have a support contract with Google, please create an issue in the [support console](https://cloud.google.com/support/) instead of filing on GitHub. This will ensure a timely response.

**Is your feature request related to a problem? Please describe.**
I was trying to create a connection with GCP and Colocated server using VPC peering. After configuring DNS, For user managed notebooks, connection works smoothly perhaps because router needs to be in the same zone in VPC peering. However, for managed notebooks there was no option to select a specific zone for example `us-west1-a` due to which it is not able to resolve name.

**Describe the solution you'd like**
I would like to see an option to select zones in the region while spinning up the managed notebooks in the vertex ai workbench. Leaving the zone automatically causes some issues when there needs to be complicated network infrastructure especially the ones which require hybrid network infra. ( GCP <-> on-prem )

L’issue ne nomme aucun fichier, test ou point d’entrée. Commencez par localiser le flux de création de managed Vertex AI Workbench et son comportement de sélection de zone, puis déterminez comment la sélection de zone doit interagir avec le provisionnement basé sur la région et validez le résultat par rapport au scénario de VPC-peering et DNS signalé.

Rédigé par le modèle d'indexation à partir du texte de l'issue.