[FEATURE] Add commands to manage sensitive variables

[vexx32]

In an upcoming version of Chocolatey Central Management we are adding functionality to define sensitive variables which can be used in deployments. We should add some commands here to manage the new API endpoints:

• Get-CcmSensitiveVariable
• Add-CcmSensitiveVariable
• Remove-CcmSensitiveVariable

Note that Get-CcmSensitiveVariable will only ever list variable names, it will not be possible to retrieve variable values via the API.