[question] do I need a real column in order to fake a computed column with MakeWrapResolversPlugin?
I wrote a MakeWrapResolversPlugin to assess whether a subtask is completable on query, since this requires some fairly involved tree processing:

```
export default makeWrapResolversPlugin({
    Query: {
        subtask: async (resolve: any) => {
            try {
                const result = await resolve()

                result.is_completable = /* custom logic here */

                return result
            } catch (e) {
                throw e
            }
        }
    }
})
```

This works great so far, but only if I have a real `is_completable` column on the table -- without one, I get an error `Cannot query field "is_completable" on type "Subtask"` on queries including the field. Is there a way I can declare `is_completable` to Postgraphile and dispense with the real column?