Allow kong mappers to define predictors

[andrewbaxter]

This allows predictors to be tied into Kong's Mappers, which can in turn be automatically attached to fields based on the field type or a type: tag. I didn't see another way to do this in kongplete, it looked like predictor attachment needed to be manually tagged in all cases.

The usage is pretty simple, if you already have a mapper just implement kongplete.PredictorProvider like:

func (_ *MyMapper) Predictor(flag *kong.Flag) complete.Predictor {
    return complete.PredictSet("val1", "val2")
}

[andrewbaxter]

@WillAbides if you have a chance, would you mind taking a look at this?