Create plugin system for expanding pyhf API

[matthewfeickert]

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Summary

Having a plugin system would allow for projects like @malin-horstmann's https://github.com/malin-horstmann/pyhf_pymc to automatically extend the pyhf API by just being installed in the same Python virtual environment as pyhf. This would also allow for the pyhf core API to exist without having to maintain lots of other extensions inside of https://github.com/scikit-hep/pyhf/ and then allow other developers to move more quickly on their own ideas.

I don't know how to create a plugin system, but https://hatch.pypa.io/latest/plugins/about/ looks like a good place to start. @kratsg if you've already got experience here please let me know.

Additional Information

N/A

Code of Conduct

• [X] I agree to follow the Code of Conduct

[matthewfeickert]

Henry has noted to me that repo-review has a section on writing plugins for it that might be useful as a starting point for comparison.