Improve powershell version selection

The help text does not represent the current state

"On Windows it uses PowerShell.exe and on Linux pwsh (PowerShell Core)"

It should be somthing like

"The used Powershell binary is defined by the tool configuration.
(DefaultWindows is used under Windows and DefaultLinux under Linux)"

Currently only these two are hardcoded with their tool name.
i.e. it is useless to create new powershell-tools because they cannot be selected.

But it would really be an added value to be able to select the tool to be used per build step.

Currently it is not possible to run scripts with Powershell5 and PowershellCore on a Windows node.
You have to choose because the "DefaultWindows" tool is always used.

Also on Linux it will be useful in the future to be able to select different versions of Powershell (6,7,8 ...).