[FEATURE] Add commands to manage sensitive variables

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.

Start by locating existing PowerShell commands in the module and the client entry points used to call Chocolatey Central Management API endpoints. Add Get-CcmSensitiveVariable, Add-CcmSensitiveVariable, and Remove-CcmSensitiveVariable, ensuring the get command lists names only; done when all three commands manage sensitive variables through the new endpoints.

Written by the indexing model from the issue text.