Update Az.Keyvault module to support new version of SecretManagement module

[dcaro]

Description of the new feature

Need to update Az.Keyvault to reflect the breaking changes introduced by preview 3:

Further details about the changes here:
https://devblogs.microsoft.com/powershell/secretmanagement-module-preview-design-changes/
https://devblogs.microsoft.com/powershell/secretmanagement-preview-3/