Add ability to create a temporary file with a specific extension to New-TemporaryFile

^{Related: #3442}

Currently, New-TemporaryFile invariably creates temp. files with extension .tmp.

Sometimes, however, it is helpful to create temp. files with a specific filename extension.

Adding an -Extension parameter could provide that:

Desired behavior

> New-TemporaryFile -Extension .ps1

    Directory: C:\Users\jdoe\AppData\Local\Temp

Mode                LastWriteTime         Length Name
----                -------------         ------ ----
-a----        7/10/2017   9:53 PM              0 tmpE3B8.ps1

Environment data

PowerShell Core v6.0.0-beta.3

[mklement0]

Whenever you need to create a temporary file to be processed by a utility that requires a specific filename extension.

Using PowerShell itself as an example, say you want to create a throw-away script dynamically, whose path you want to pass to another PowerShell instance via the CLI (powershell -File ...).
Such a throw-away script only works if it has extension *.ps1.

[bergmeister]

Well it’s currently on track for .Net 5, meaning we could get it maybe at the end of this year for PS 7.1 if the API arrives in time (the change in PowerShell should be quite small once the API is there) https://github.com/dotnet/runtime/issues/2048