Extend cmdlet Get-IShEvent with IshBackgroundTask parameter overload as Add-IshBackgroundTask companion

[ddemeyer]

The request is to make it possible to easily catch and follow-up on the Events generated of a new BackgroundTask (see recent #193)

Add-IShBackgroundTask ... | Get-IShEvent
#or
$justCreatedBackgroundTask = Add-IShBackgroundTask ...
$justCreatedBackgroundTask | Get-IShEvent
Get-IshEvent -IshBackgroundTask $justCreatedBackgroundTask

• Start from ParameterSetName IshEventsGroup, same parameters like EventLevel, MetadataFilter, RequestedMetadata and of course IshSession
• Just like -IshEvent is an array, so is -IshBackgroundTask ... if you do a group request you just get them concatenated as already implemented when offering multiple inputs on -IshEvent ... For IshEvent you go to _retrievedIshEvents , but for IshBackgroundTask you can immediately skip to progressRefs which we should rename to a class property _retrievedProgressRefs as they can already be filled up from the ProcessRecord function
• Extend inline tripple-slash documentation for Get-Help usage
• Add Pester test

/// <summary>
/// <para type="description">The <see cref="IshEvent"/>s that need to be handled.</para>
/// </summary>
[Parameter(Mandatory = true, ValueFromPipeline = true, ParameterSetName = "IshEventsGroup")]
public IshEvent[] IshEvent { get; set; }