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

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**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 )

この issue では、ファイル、テスト、エントリーポイントが特定されていません。まず managed Vertex AI Workbench の作成フローとゾーン選択の動作を見つけ、次にゾーン選択がリージョンベースのプロビジョニングとどのように連携すべきかを判断し、その結果を報告された VPC-peering と DNS のシナリオに照らして検証してください。

索引モデルが issue の本文から書いたものです。