[btown]

> returns a reusable data structure mapping input keys to output keys

IMO this use case is exactly what Copilot is for. Write a comment including one example each of input and output, and tab-complete in your language of choice to have it create a rewriter for you.

One benefit (and danger) is that it will look at the values, not just the keys, and also may generate arbitrary code that can e.g. adapt a firstName and lastName to a fullName. But that's why you have a human being triggering and auditing this for subtle bugs, and putting it through code review and source control, right?