[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.

**Information Checklist**
Kindly make sure that you have added all the following information above and checkoff the required fields otherwise we will treat the issuer as an incomplete report
- [X] Description Added
- [X] Expected solution specified

Start at the ContainerGroup definition and fluent API, then compare the requested withVnet() and withSubnet() builders with Microsoft's Azure Container Instances existing-virtual-network documentation. Identify the relevant client-library model and validation tests, and consider the feature complete when an existing virtual network and subnet can be represented in a ContainerGroup definition and exercised through the Java API.

Scritto dal modello di indicizzazione a partire dal testo della issue.