[AKS] Add support for updating cluster credential

[dcaro]

Description of the new feature

Enable update of AKS cluster credentials from PowerShell cmdlets.

1. Update cluster with new Service Principal credentials
2. Update AKS cluster with new AAD application credentials

https://docs.microsoft.com/en-us/azure/aks/update-credentials

Proposed implementation details (optional)

[wyunchi-ms]

This issue is supported in [#14821]. But still need to verify it.