Connect-MgGraph: Environment parameter does not support auto-complete

[adamedx]

The Environment parameter of Connect-MgGraph requires the user to use Get-MgEnvironment to enumerate environments. Instead, we should add a parameter completer that does this so the user gets auto-complete.

We should also add this completer to any other commands in Microsoft.Graph.Authentication that take Environment as a parameter, such as Get-MgEnvironment.

[FehintolaObafemi]

https://github.com/microsoftgraph/msgraph-sdk-powershell/tree/fehintolaobafemi/parameter-completers