feat: add cabinetry to StatAnalysis

Adding cabinetry to StatAnalysis.

• documentation: https://cabinetry.readthedocs.io/
• GitHub: https://github.com/scikit-hep/cabinetry/

I also added a test for functionality, which currently uses a small (16 KB) ROOT file with histograms. Let me know if you prefer having that in LFS, or if you would rather have the file generated dynamically (also an option, I have a script for it).

More context: cabinetry provides two kinds of functionality: workspace construction & statistical inference. It heavily relies on pyhf for this (building workspace in the pyhf JSON format and using such workspaces for inference). The functionality is similar to other integrated tools like TRExFitter, but it is a library (while TRExFitter is a framework) providing you the building blocks to write the high level code that calls the API.