Add openapi.json to the repo

[patrick-austin]

Possibly using nox, or similar, we should have a method to generate the openapi.json file and fail the GHA if it has not been updated to reflect changes to the source code. E.G. If I add a new endpoint, but do not generate the new json, the CI should generate a new file, diff it against the previous version and fail if there are any changes. So that the developer needs to have generated the new one before pushing their change for the CI to pass.

Should also check other FastAPI repos developed by the group, and see if they already use some method for this that we can re-use.