ActiveDirectoryDsc: Use New-CimCredentialInstance for credential objects

[johlju]

We should use the same pattern as in xADDomainTrust to return the credential objects from Get-TargetResource.

https://github.com/PowerShell/xActiveDirectory/blob/dd53b43a46a6e7ddd101a0b7f393af0d377b100e/DSCResources/MSFT_xADDomainTrust/MSFT_xADDomainTrust.psm1#L60-L66

The helper function New-CimCredentialInstance will create a MSFT_Credential CIM instance credential object without a password that can be used to return the actual username used from GetTargetResource.

This should be used for all resource that have a MSFT_Credential instance in the schema.mof.