[KeyVault] Support for soft-delete on/off/null as a one time operation

[dingmeng-xue]

Description of the new feature

Keyvault team are changing the default behavior that when creating a new keyvault, it's soft-delete is on.
We need to make sure:

• The default behavior remains the same (soft delete is off) so as to keep it compatible
• Provide a one-time operation to enable soft delete (this cannot be undone)

Proposed implementation details (optional)

Since the requirement is changing a property of the keyvault resource, idealy we should add an optional parameter [-EnableSoftDelete] to the Update-AzKeyVault cmdlet, but that cmdlet does not exist (only get- new- remove-), so we might need to add a update- cmdlet.

• .Net SDK
  • Update api version
  • Write scenario tests
• PowerShell
  • Utilize new SDK
  • Keep the current behavior of New-AzKeyVault
  • A new cmdlet: Update-AzKeyVault
  • Tests

[isra-fel]

Scenario tests

• .NET SDK
  • Create a new keyvault without enableSoftDelete, the keyvault should enable soft-delete
  • Create a new keyvault with enableSoftDelete = false, the keyvault should not enable soft-delete
• PowerShell cmdlet
  • New-AzKeyVault without -EnableSoftDelete, the keyvault should not enable soft-delete
  • New-AzKeyVault with -EnableSoftDelete, the keyvault should enable soft-delete
  • Update-AzKeyVault with -EnableSoftDelete, if it's not enabled, the keyvault should enable soft-delete
  • Update-AzKeyVault with -EnableSoftDelete, if it's already enabled, the keyvault should prompt a warning

[isra-fel]

Azure/azure-sdk-for-net#8956