[FEATURE REQ] Deploy Azure container instances into virtual network

**Is your feature request related to a problem? Please describe.**
We'd like to be able to deploy container groups into an existing virtual network using the fluent API. The feature exists but doesn't appear to be supported in Java yet:
https://docs.microsoft.com/en-us/azure/container-instances/container-instances-vnet#deploy-to-existing-virtual-network

**Describe the solution you'd like**
As part of the ContainerGroup definition, add new builder functions .withVnet().withSubnet().

**Describe alternatives you've considered**
Thought about making a system call out to the Azure CLI tool to avoid this library entirely - not viable for my use case.

**Additional context**
When Microsoft publishes new features into preview, it would be great to know the roadmap for when we can expect to be able to use them in the client libs.

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