ARO-13921 Add cluster subnets' network security groups to List Cluster Azure Resources output if cluster uses preconfiguredNSG

[aasserzo]

Which issue this PR addresses:

Fixes ARO-13921

What this PR does / why we need it:

Currently the List Cluster Azure Resources admin endpoint lists all resources within the managed resource group.
BYO NSGs, that some customers might have configured, belong to different resource groups, and are therefore omitted from this endpoint's output.
This change checks whether a subnet has an NSG that belongs to a different RG, and appends it to the Cluster Resources list.

Test plan for issue:

The test mocks two subnets, one containing a managed NSG, the other containing a BYO NSG, then runs WriteToStream().
That, in turn, calls appendAzureNetworkResources().
The expected result is for both of them to be present in the resulting list.

Is there any documentation that needs to be updated for this PR?

No documentation exists for this functionality.

How do you know this will function as expected in production?

TBD

[mrWinston]

Looking really good! I mostly have minor comments regarding style. the only important thing for me would be the nil check for snet.Properties.NetworkSecurityGroup.