Update Connect-AzureADExporter.ps1 with dynamic tab autocomplete for environment name

[SamErde]

Used ArgumentCompleter (backwards compatible with Windows PowerShell 5.1) to dynamically pull Azure environment names for tab autocomplete of the Environment parameter.

Resolves issue #22 (tab autocomplete for Azure environment names should not be hard coded).

[SamErde]

That commit gives us dynamic tab autocomplete, but I just realized that it doesn't validate inputs. Would this be better?

function Connect-AzureTestCompatible {
    [CmdletBinding()]
    Param(
        [Parameter(Mandatory=$false)]
        [ArgumentCompleter( {
            param ( $CommandName, $ParameterName, $WordToComplete, $CommandAst, $FakeBoundParameters )
            $Global:EnvironmentNames = (Get-MgEnvironment).Name; $EnvironmentNames
        } )]
        [ValidateScript({
            if ($_ -in $Global:EnvironmentNames) {
                $true
            } else {
                throw "$_ is not a valid environment name. Please use one of the following: $($Global:EnvironmentNames -join ', ')"
            }
        })]
        [string]$Environment
    )
    write-output $Environment
}