Issue title: xSQLServerFirewall: SourceCredential for SourcePath not used
 Issue body: xSQLServerFirewall doesn't pass SourceCredential when it calls Get-TargetResource, so it can't access the setup executable:

https://github.com/PowerShell/xSQLServer/blob/1bbdecaf5f75245c2b16e698579b28bf90bcc240/DSCResources/MSFT_xSQLServerFirewall/MSFT_xSQLServerFirewall.psm1#L384

https://github.com/PowerShell/xSQLServer/blob/1bbdecaf5f75245c2b16e698579b28bf90bcc240/DSCResources/MSFT_xSQLServerFirewall/MSFT_xSQLServerFirewall.psm1#L606

Also, at a quick glance it seems like the only purpose for SourcePath is to check the SQL Server version. Would this be better done through the Registry?
 Comments: 
 Comment 0: Good catch. Label this as a bug.

Regarding SQL Version, moving that to a separate issue #862.


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