Keep generated files up to date

[cmester0]

Since people are beginning to look at the generated files, we should have some way to keep these up to date, preferably without cluttering pull requests with updates to generated files. #232 may be relevant for this too.

[tarcieri]

One option is to have a GitHub Action which regenerates the files, which can be triggered when any of the input files are touched.

It can run the codegen and open a PR if any of the generated files have been changed.

[franziskuskiefer]

👍🏻 yes, we should put the code generation into a GH action. This will need some setup work that's necessary for running backend tests anyway. There's #154 that I meant to tackle for a while ...