Gitlab CI: Windows Configs (#43967)

Add support for Gitlab CI on Windows

This PR adds the config changes required to configure and execute
Gitlab pipelines running Windows builds on Windows runners using
the existing Gitlab CI infrastructure (and newly added Windows 
infrastructure).

* Adds support for generating child pipelines dispatched to Windows runners
* Refactors the relevant pre-scripts, scripts, and post scripts to be compatible with Windows
* Adds Windows config section describing Windows jobs
* Adds VTK as Windows build stack (to be expanded later)
* Modifies proj to build on Windows
* Refactors Windows rpath symlinking to avoid system libs and externals

---------

Co-authored-by: Ryan Krattiger <ryan.krattiger@kitware.com>
Co-authored-by: Mike VanDenburgh <michael.vandenburgh@kitware.com>
Co-authored-by: Todd Gamblin <tgamblin@llnl.gov>
Co-authored-by: Scott Wittenburg <scott.wittenburg@kitware.com>

bin/spack.ps1

@@ -144,3 +144,5 @@ switch($SpackSubCommand)
     "unload" {Invoke-SpackLoad}
     default  {python "$Env:SPACK_ROOT/bin/spack" $SpackCMD_params $SpackSubCommand $SpackSubCommandArgs}
 }
+
+exit $LASTEXITCODE