add network describer interface

[alexeldeib]

/kind cleanup

Describe the solution you'd like
This is a convenient way to allow for multiple infrastructure types to implement the necessary interfaces (ClusterDescriber and NetworkDescriber) to describe all required Azure infrastructure and secrets.

It's a stepping stone to removing AzureCluster from MachineScope, so MachineScope can use different Cluster/ControlPlane providers

Anything else you would like to add:
#822 (comment)

Environment:

• cluster-api-provider-azure version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

[CecileRobertMichon]

/assign @alexeldeib
/assign

Just assigning us for now so no one else starts working on it while we figure out how to minimize merge conflicts. If you don't open a PR for it, I can after I finish with VMs.