Improve GVK selection logic for VirtualNetwork, RouteTable, and LoadBalancer extensions

[super-harsh]

We have a few resource extensions, where we need to merge the child resource into the parent resource into the ARM payload. These resource relationships are:

• VirtualNetwork - VirtualNetworksSubnet
• LoadBalancer - LoadBalancersInboundNatRule
• RouteTable - RouteTablesRoute

Currently, how we get the GVK for a child resource off from the parent resource in these extensions is not an ideal way to fetch GVK for a resource as we may run into problems when there are multiple versions involved. More discussion on comment

[matthchr]

We should still do this - what we have here is a bit fragile, but we do have tests that will catch if it breaks

[matthchr]

This is still fragile, but we do have tests

[matthchr]

No change from above

[matthchr]

No change from above

[theunrepentantgeek]

Still interested in finding a better approach.

[matthchr]

No change from above - the fact that this hasn't caused issues in a few years makes me think it's probably not that fragile (even though it is ugly).