Add Credentials capability to SHiPSProvider.

[yurko7]

It should be much easier to build providers for data sources that require authentication if add Credentials capability to SHiPSProvider.
Also Drive (PSDriveInfo) property should be added to the ProviderContext class to provide authors access to credentials specified by user in -Credential parameter of New-PSDrive cmdlet.

[ZenoArrow]

Is this issue now resolved? It's not clear to me based on the associated pull request.