Putting Container Group in subnet is only possible via deprecated 'NetworkProfile'

[wkoeter]

Is your feature request related to a problem? Please describe.
Network profiles are deprecated according to MS documentation
https://docs.microsoft.com/en-us/azure/container-instances/container-instances-vnet

It is the only way using the fluent SDK to add a container group to a VNET, and lacks good documentation (because it is deprecated maybe?).
WithNetworkProfileId(subscriptionId, resourceGroupName, NetworkProfileName)

Describe the solution you'd like
AZ CLI and ARM templates do not need to add these anymore to integrate a container group into a vnet. Please create an alternative way (e.g. subnet id) to incorporate a container group into a vnet that does not use deprecated concepts.

AttachToSubnet(resourceGroupName, virtualNetworkName, subnetName)

Describe alternatives you've considered
Not using the SDK, but its another team that is using it so that would not be a preferred choice at all.

Additional context
Azure container group gets created from an application and needs to be deployed in a vnet using the SDK. Only NetworkProfiles allow this, but they are deprecated and poorly documented.

Information Checklist
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[weidongxu-microsoft]

For .NET, the development focus has shifted to the next generation of Azure SDKs which follows the new SDK guideline and introduces a set of important new features. Those new packages are currently in preview state and we are actively working on making it production ready. You can visit this link here to see the latest .NET packages:
https://devblogs.microsoft.com/azure-sdk/october-2020-management-ga/

We have also published a few blog posts on why we are doing this:
https://devblogs.microsoft.com/azure-sdk/introducing-new-previews-for-azure-management-libraries/

With this background, .NET Fluent is currently in a low maintenance mode, and we mostly do security and bug fixes. It is subject to deprecation in the future when the new set of .NET packages become Generally Available (GA). Please let us know if there are further questions, thanks!

[wkoeter]

As mentioned in #1291 , the container instance is not covered in new SDK. For now I will use the non-fluent version of the old SDK since that is more flexible and appears more up to date with latest API.