Allow `azd ai agent` to use a private-VNet Foundry account (provision new or reuse an existing account)

We need a way to support private vnets, which requires settings on the account - its not something that can be changed later. 

https://github.com/Azure-Samples/azd-ai-starter-basic/blob/main/infra/core/ai/ai-project.bicep#L108-L140

At a minimum, we need to at least support existing accounts so devs could use one that is already setup for a vnet.