feat!: (IAC-1009) Add support for network plugin mode overlay

[riragh]

Changes:

This PR adds support to Azure CNI Overlay. New variable aks_network_plugin_mode is introduced in this change with possible value of "overlay". For more details see Azure CNI Overlay.

The codebase was updated to change the default value for aks_network_policy to null, as Azure CNI Overlay supports network policy options of "azure", "calico", or null.
This changes the previous default value for aks_network_policy, which would result in breaking change for existing users.

Test:

Verified following scenarios, see internal ticket for details:

Scenario	Provider	Task	kubectl version	order	cadence
1	Azure	aks_cluster_sku_tier = free, aks_network_plugin = "azure", aks_network_policy = "azure", aks_network_plugin_mode = "overlay"	1.27.9	******	stable:2023.12
2	Azure	aks_cluster_sku_tier = Standard, aks_network_plugin = "azure", aks_network_policy = "azure", aks_network_plugin_mode = "overlay"	1.27.9	******	stable:2023.12
3	Azure	aks_cluster_sku_tier = free, aks_network_plugin = "azure", aks_network_plugin_mode = "overlay"	1.27.9	******	stable:2023.12
4	Azure	aks_cluster_sku_tier = Standard, aks_network_plugin = "azure", aks_network_plugin_mode = "overlay"	1.27.9	******	stable:2023.12
5	Azure	aks_cluster_sku_tier = free, aks_network_plugin = "azure", aks_network_policy = "calico", aks_network_plugin_mode = "overlay"	1.27.9	******	stable:2023.12
6	Azure	aks_cluster_sku_tier = Standard, aks_network_plugin = "azure", aks_network_policy = "calico", aks_network_plugin_mode = "overlay"	1.27.9	******	fast:2020

[thpang]

LGTM

[sayeun]

👍