Use git to determine and export version tags automatically

With this change, you don't have to worry about keeping a version number synchronized between your build system, your release versioning, and your git tags. Just tag a gitlab release v0.0.1 and it will automatically show up in CMake and when exported for downstream projects. @Andrew @wiarda I'll walk you through this when we merge this if you're up for it.

This script is used in Lava, Celeritas, ForTrilinos, and other projects I manage.