Support cross platform pwsh in powershell_exec

[mwrock]

Related to epic #10330

Tentatively users of powershell_exec may specify an "interpreter" as an optional second string argument to powershell_exec. The default will be powershell. If pwsh is the interpreter, powershell exec will invoke a .net core based wrapper for creating a cross platform powershell based runspace.

"interpreter" may not be the correct path forward here so that implementation detail is open. We could also support a boolean flag.