WorkflowInstance cmdlet improvements

[fowl2]

Improvements to *-WorkflowInstance cmdlets

Get-WorkflowInstance:

• add OutputType annotation
• add getting site workflow instances
• add get by instance guid
• positional and piped list item and subscription parameters
• get by list item object without specifying a list pipebind
• added extra examples

Resume-WorkflowInstance:

• improved implementation to retrieve just the requested instance instead of getting all and filtering
• added examples

Start-WorkflowInstance:

• return the Guid of the started instance
• added extra examples
• made list item optional, allowing start of site workflows
• positional and piped subscription and list item parameter
• added Initiator and InitiationParameters parameters

Stop:-WorkflowInstance:

• added extra example
• positional and piped instance parameter
• verbose message now indicates cancel vs terminate

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edit: The diffs look a bit better in Visual Studio